I want to know what exactly is sequential write and what is random write in definition. I will be even more helpful with example. I tried to google the result. But not much google explanation.
Thanks
When you write two blocks that are next to each-other on disk, you have a sequential write.
When you write two blocks that are located far away from eachother on disk, you have random writes.
With a spinning hard disk, the second pattern is much slower (can be magnitudes), because the head has to be moved around to the new position.
Database technology is (or has been, maybe not that important with SSD anymore) to a large part about optimizing disk access patterns. So what you often see, for example, is trading direct updates of data in their on-disk location (random access) versus writing to a transaction log (sequential access). Makes it more complicated and time-consuming to reconstruct the actual value, but makes for much faster commits (and you have checkpoints to eventually consolidate the logs that build up).
I have a program that is used to exercise several disk units in a raid configuration. 1 process synchronously (O_SYNC) writes random data to a file using write(). It then puts the name of the directory into a shared-memory queue, where a 2nd process is waiting for the queue to have entries to read the data back into memory using read().
The problem that I can't seem to overcome is that when the 2nd process attempts to read the data back into memory, none of the disk units show read accesses. The program has code to check whether or not the data read back in is equal to the code that is written to disk, and the data always matches.
My question is, how can I make the OS (IBM i) not buffer the data when it is written to disk so that the read() system call accesses the data on the disk rather than in cache? I am doing simple throughput calculations and the read() operations are always 10+ times faster than the write operations.
I have tried using the O_DIRECT flag, but cannot seem to get the data to write to the file. It could have to do with setting up the correct aligned buffers. I have also tried the posix_fadvise(fd, offset,len, POSIX_FADV_DONTNEED) system call.
I have read through this similar question but haven't found a solution. I can provide code if it would be helpful.
My though is that if you write ENOUGH data, then there simply won't be enough memory to cache it, and thus SOME data must be written to disk.
You can also, if you want to make sure that small writes to your file works, try writing ANOTHER large file (either from the same process or a different one - for example, you could start a process like dd if=/dev/zero of=myfile.dat bs=4k count=some_large_number) to force other data to fill the cache.
Another "trick" may be to "chew up" some (more like most) of the RAM in the system - just allocate a large lump of memory, then write to some small part of it at a time - for example, an array of integers, where you write to every 256th entry of the array in a loop, moving to one step forward each time - that way, you walk through ALL of the memory quickly, and since you are writing continuously to all of it, the memory will have to be resident. [I used this technique to simulate a "busy" virtual machine when running VM tests].
The other option is of course to nobble the caching system itself in OS/filesystem driver, but I would be very worried about doing that - it will almost certainly slow the system down to a slow crawl, and unless there is an existing option to disable it, you may find it hard to do accurately/correctly/reliably.
...exercise several disk units in a raid configuration... How? IBM i doesn't allow a program access to the hardware. How are you directing I/O to any specific physical disks?
ANSWER: The write/read operations are done in parallel against IFS so the stream file manager is selecting which disks to target. By having enough threads reading/writing, the busyness of SYSBASE or an IASP can be driven up.
...none of the disk units show read accesses. None of them? Unless you are running the sole job on a system in restricted state, there is going to be read activity on the disks from other tasks. Is the system divided into multiple LPARs? Multiple ASPs? I'm suggesting that you may be monitoring disks that this program isn't writing to, because IBM i handles physical I/O, not programs.
ANSWER I guess none of them is a slight exaggeration - I know which disks belong to SYSBASE and those disks are not being targeted with many read requests. I was just trying to generalize for an audience not familiar w/IBM i. In the picture below, you will see that the write reqs are driving the % busyness up, but the read reqs are not even though they are targeting the same files.
...how can I make the OS (IBM i) not buffer the data when it is written to disk... Use a memory starved main storage pool to maximise paging, write immense blocks of data so as to guarantee that the system and disk controller caches overflow and use a busy machine so that other tasks are demanding disk I/O as well.
I have a program that processes a large dataset consisting of a large number (300+) of sizable memory (40MB+) mapped files. All the files are needed together though they are accessed in a sequential way. at the moment I am memory mapping the files and then using madvise with MADV_SEQUENTIAL since I don't want the thing to be any more of a memory hog than it needs to be (without any madvise the consumption becomes a problem). The problem it that the program runs much slower (like 50x slower) than the diskio of the system would indicate it should, and becomes worse faster than linearly. as the number of files are involved are increased. Processing 100 files is more than 10x faster than 300 files despite being only 3x the data. I suspect that the memory mapped files are generating a page fault every time a 4kb page is crossed, net result disk seek time is greater than disk transfer time.
Can anyone think of a better way than using madvise with MADV_WILLNEED and MADV_DONTNEED every so often, and if this is the best way, any ideas as to how far to look ahead?
I've run into a situation where lseek'ing forward repeatedly through a 500MB file and reading a small chunk (300-500 bytes) between each seek appears to be slower than read'ing through the whole file from the beginning and ignoring the bytes I don't want. This appears to be true even when I only do 5-10 seeks (so when I only end up reading ~1% of the file). I'm a bit surprised by this -- why would seeking forward repeatedly, which should involve less work, be slower than reading which actually has to copy the data from kernel space to userspace?
Presumably on local disk when seeking the OS could even send a message to the drive to seek without sending any data back across the bus for even more savings. But I'm accessing a network mount, where I'd expect read to be much slower than seek (sending one packet saying to move N bytes ahead versus actually transferring data across the network).
Regardless of whether reading from local disk or a network filesystem, how could this happen? My only guess is the OS is prefetching a ton of data after each location I seek to. Is this something that can normally occur or does it likely indicate a bug in my code?
The magnitude of the difference will be a factor of the ratio of the seek count/data being read to the size of the entire file.
But I'm accessing a network mount, where I'd expect read to be much slower than seek (sending one packet saying to move N bytes ahead versus actually transferring data across the network).
If there's rotational magnetic drives at the other end of the network, the effect will still be present and likely significantly compounded by the round trip time. The network protocol may play a role too. Even solid state drives may take some penalty.
I/O schedulers may reorder requests in order to minimize head movements (perhaps naively even for storage devices without a head). A single bulk request might give you some greater efficiency across many layers. The filesystems have an opportunity to interfere here somewhat.
Regardless of whether reading from local disk or a network filesystem, how could this happen?
I wouldn't be quick to dismiss the effect of those layers -- do you have measurements which show the same behavior from a local disk? It's much easier to draw conclusions without quite so much between you and the hardware. Start with a raw device and bisect from there.
Have you considered using a memory map instead? It's perfect for this use case.
Depending on the filesystem, the specific lseek implementation make create some overhead.
For example, I believe when using NFS, lseek locks the kernel by calling remote_llseek().
I have a thread that needs to write data from an in-memory buffer to a disk thousands of times. I have some requirements of how long each write takes because the buffer needs to be cleared for a separate thread to write to it again.
I have tested the disk with dd. I'm not using any filesystem on it and writing directly to the disk (opening it with the direct flag). I am able to get about 100 MB/s with a 32K block size.
In my application, I noticed I wasn't able to write data to the disk at nearly this speed. So I looked into what was happening and I find that some writes are taking very long. My block of code looks like (this is in C by the way):
last = get_timestamp();
write();
now = get_timestamp();
if (longest_write < now - last)
longest_write = now - last;
And at the end I print out the longest write. I found that for a 32K buffer, I am seeing a longest write speed of about 47ms. This is way too long to meet the requirements of my application. I don't think this can be solely attributed to rotational latency of the disk. Any ideas what is going on and what I can do to get more stable write speeds? Thanks
Edit:
I am in fact using multiple buffers of the type I declare above and striping between them to multiple disks. One solution to my problem would be to just increase the number of buffers to amortize the cost of long writes. However I would like to keep the amount of memory being used for buffering as small as possible to avoid dirtying the cache of the thread that is producing the data written into the buffer. My question should be constrained to dealing with variance in the latency of writing a small block to disk and how to reduce it.
I'm assuming that you are using an ATA or SATA drive connected to the built-in disk controller in a standard computer. Is this a valid assumption, or are you using anything out of the ordinary (hardware RAID controller, SCSI drives, external drive, etc)?
As an engineer who does a lot of disk I/O performance testing at work, I would say that this sounds a lot like your writes are being cached somewhere. Your "high latency" I/O is a result of that cache finally being flushed. Even without a filesystem, I/O operations can be cached in the I/O controller or in the disk itself.
To get a better view of what is going on, record not just your max latency, but your average latency as well. Consider recording your max 10-15 latency samples so you can get a better picture of how (in-)frequent these high-latency samples are. Also, throw out the data recorded in the first two or three seconds of your test and start your data logging after that. There can be high-latency I/O operations seen at the start of a disk test that aren't indicative of the disk's true performance (can be caused by things like the disk having to rev up to full speed, the head having to do a large initial seek, disk write cache being flushed, etc).
If you are wanting to benchmark disk I/O performance, I would recommend using a tool like IOMeter instead of using dd or rolling your own. IOMeter makes it easy to see what kind of a difference it makes to change the I/O size, alignment, etc, plus it keeps track of a number of useful statistics.
Requiring an I/O operation to happen within a certain amount of time is a risky thing to do. For one, other applications on the system can compete with you for disk access or CPU time and it is nearly impossible to predict their exact effect on your I/O speeds. Your disk might encounter a bad block, in which case it has to do some extra work to remap the affected sectors before processing your I/O. This introduces an unpredictable delay. You also can't control what the OS, driver, and disk controller are doing. Your I/O request may get backed up in one of those layers for any number of unforseeable reasons.
If the only reason you have a hard limit on I/O time is because your buffer is being re-used, consider changing your algorithm instead. Try using a circular buffer so that you can flush data out of it while writing into it. If you see that you are filling it faster than flushing it, you can throttle back your buffer usage. Alternatively, you can also create multiple buffers and cycle through them. When one buffer fills up, write that buffer to disk and switch to the next one. You can be writing to the new buffer even if the first is still being written.
Response to comment:
You can't really "get the kernel out of the way", it's the lowest level in the system and you have to go through it to one degree or another. You might be able to build a custom version of the driver for your disk controller (provided it's open source) and build in a "high-priority" I/O path for your application to use. You are still at the mercy of the disk controller's firmware and the firmware/hardware of the drive itself, which you can't necessarily predict or do anything about.
Hard drives traditionally perform best when doing large, sequential I/O operations. Drivers, device firmware, and OS I/O subsystems take this into account and try to group smaller I/O requests together so that they only have to generate a single, large I/O request to the drive. If you are only flushing 32K at a time, then your writes are probably being cached at some level, coalesced, and sent to the drive all at once. By defeating this coalescing, you should reduce the number of I/O latency "spikes" and see more uniform disk access times. However, these access times will be much closer to the large times seen in your "spikes" than the moderate times that you are normally seeing. The latency spike corresponds to an I/O request that didn't get coalesced with any others and thus had to absorb the entire overhead of a disk seek. Request coalescing is done for a reason; by bundling requests you are amortizing the overhead of a drive seek operation over multiple commands. Defeating coalescing leads to doing more seek operations than you would normally, giving you overall slower I/O speeds. It's a trade-off: you reduce your average I/O latency at the expense of occasionally having an abnormal, high-latency operation. It is a beneficial trade-off, however, because the increase in average latency associated with disabling coalescing is nearly always more of a disadvantage than having a more consistent access time is an advantage.
I'm also assuming that you have already tried adjusting thread priorities, and that this isn't a case of your high-bandwidth producer thread starving out the buffer-flushing thread for CPU time. Have you confirmed this?
You say that you do not want to disturb the high-bandwidth thread that is also running on the system. Have you actually tested various output buffer sizes/quantities and measured their impact on the other thread? If so, please share some of the results you measured so that we have more information to use when brainstorming.
Given the amount of memory that most machines have, moving from a 32K buffer to a system that rotates through 4 32K buffers is a rather inconsequential jump in memory usage. On a system with 1GB of memory, the increase in buffer size represents only 0.0092% of the system's memory. Try moving to a system of alternating/rotating buffers (to keep it simple, start with 2) and measure the impact on your high-bandwidth thread. I'm betting that the extra 32K of memory isn't going to have any sort of noticeable impact on the other thread. This shouldn't be "dirtying the cache" of the producer thread. If you are constantly using these memory regions, they should always be marked as "in use" and should never get swapped out of physical memory. The buffer being flushed must stay in physical memory for DMA to work, and the second buffer will be in memory because your producer thread is currently writing to it. It is true that using an additional buffer will reduce the total amount of physical memory available to the producer thread (albeit only very slightly), but if you are running an application that requires high bandwidth and low latency then you would have designed your system such that it has quite a lot more than 32K of memory to spare.
Instead of solving the problem by trying to force the hardware and low-level software to perform to specific performance measurements, the easier solution is to adjust your software to fit the hardware. If you measure your max write latency to be 1 second (for the sake of nice round numbers), write your program such that a buffer that is flushed to disk will not need to be re-used for at least 2.5-3 seconds. That way you cover your worst-case scenario, plus provide a safety margin in case something really unexpected happens. If you use a system where you rotate through 3-4 output buffers, you shouldn't have to worry about re-using a buffer before it gets flushed. You aren't going to be able to control the hardware too closely, and if you are already writing to a raw volume (no filesystem) then there's not much between you and the hardware that you can manipulate or eliminate. If your program design is inflexible and you are seeing unacceptable latency spikes, you can always try a faster drive. Solid-state drives don't have to "seek" to do I/O operations, so you should see a fairly uniform hardware I/O latency.
As long as you are using O_DIRECT | O_SYNC, you can use ioprio_set() to set the IO scheduling priority of your process/thread (although the man page says "process", I believe you can pass a TID as given by gettid()).
If you set a real-time IO class, then your IO will always be given first access to the disk - it sounds like this is what you want.
I have a thread that needs to write data from an in-memory buffer to a disk thousands of times.
I have tested the disk with dd. I'm not using any filesystem on it and writing directly to the disk (opening it with the direct flag). I am able to get about 100 MB/s with a 32K block size.
The dd's block size is aligned with file system block size. I guess your log file isn't.
Plus probably your application writes not only the log file, but also does some other file operations. Or your application isn't alone using the disk.
Generally, disk I/O isn't optimized for latencies, it is optimized for the throughput. High latencies are normal - and networked file systems have them even higher.
In my application, I noticed I wasn't able to write data to the disk at nearly this speed. So I looked into what was happening and I find that some writes are taking very long.
Some writes take longer time because after some point of time you saturate the write queue and OS finally decides to actually flush the data to disk. The I/O queues by default configured pretty short: to avoid excessive buffering and information loss due to a crash.
N.B. If you want to see the real speed, try setting the O_DSYNC flag when opening the file.
If your blocks are really aligned you might try using the O_DIRECT flag, since that would remove contentions (with other applications) on the Linux disk cache level. The writes would work at the real speed of the disk.
100MB/s with dd - without any syncing - is a highly synthetic benchmark, as you never know that data have really hit the disk. Try adding conv=dsync to the dd's command line.
Also trying using larger block size. 32K is still small. IIRC 128K size was the optimal when I was testing sequential vs. random I/O few years ago.
I am seeing a longest write speed of about 47ms.
"Real time" != "fast". If I define max response time of 50ms, and your app consistently responds within the 50ms (47 < 50) then your app would classify as real-time.
I don't think this can be solely attributed to rotational latency of the disk. Any ideas what is going on and what I can do to get more stable write speeds?
I do not think you can avoid the write() delays. Latencies are the inherit property of the disk I/O. You can't avoid them - you have to expect and handle them.
I can think only of the following option: use two buffers. First would be used by write(), second - used for storing new incoming data from threads. When write() finishes, switch the buffers and if there is something to write, start writing it. That way there is always a buffer for threads to put the information into. Overflow might still happen if threads generate information faster than the write() can write. Dynamically adding more buffers (up to some limit) might help in the case.
Otherwise, you can achieve some sort of real-time-ness for (rotational) disk I/O only if your application is the sole user of the disk. (Old rule of real time applications applies: there can be only one.) O_DIRECT helps somehow to remove the influence of the OS itself from the equation. (Though you would still have the overhead of file system in form of occasional delays due to block allocation for the file extension. Under Linux that works pretty fast, but still can be avoided by preallocating the whole file in advance, e.g. by writing zeros.) If the timing is really important, consider buying dedicated disk for the job. SSDs have excellent throughput and do not suffer from the seeking.
Are you writing to a new file or overwriting the same file?
The big difference with dd is likely to be seek time, dd is streaming to a contigous (mostly) list of blocks, if you are writing lots of small files the head may be seeking all over the drive to allocate them.
The best way of solving the problem is likely to be removing the requirement for the log to be written in a specific time. Can you use a set of buffers so that one is being written (or at least sent to the drives's buffer) while new log data is arriving into another one?
linux does not write anything directly to the disk it will use the virtual memory and then, a kernel thread call pdflush will write these datas to the disk , the behavior of pdflush could be controlled through sysctl -w ""