Re: [ceph-users] Poor performance for 512b aligned "partial" writes from Windows guests in OpenStack + potential fix
For libvirt VMs, first you need to add "" to the relevant sections, and then
stop/start the VM to apply the change.
Then you need to make sure your VirtIO drivers (the Fedora/Red Hat variety
anyway) are from late 2018 or so. There was a bug fixed around July 2018,
before that date, the physical_block_size=4096 parameter is not used by the
Windows VirtIO driver (it was supposed to be, but did not work).
Relevant links:
https://bugzilla.redhat.com/show_bug.cgi?id=1428641
https://github.com/virtio-win/kvm-guest-drivers-windows/pull/312
After that, you can check if Windows is correctly recognizing the physical
block size,
Start cmd.exe with "Run as administrator", then run
fsutil fsinfo ntfsinfo c:
It should show "Bytes Per Physical Sector : 4096"
Lastly at least for Windows itself this makes it do 4096-byte writes "most
of the time", however some applications including Exchange have special
handling of the sector size. I'm not really sure how MSSQL handles it, for
example, it may or may not work correctly if you switch to 4096 bytes after
installation - you may have to create new data files or something for it to
do 4k segments - or not. Hopefully the MSSQL documentation has some
information about that.
It is also possible to set logical_block_size=4096 as well as
physical_block_size=4096 ("4k native") however this absolutely causes
problems with some software (e.g. exchange) if you convert an existing
installation between the two. If you try to use 4k native mode, ideally you
would want to do a fresh install, to avoid any such issues. Or again, refer
to the docs and test it. Just beware it may cause issues if you try to
switch to 4k native.
As a final note you can use this tool to process an OSD log with "debug
filestore = 10" enabled, it will print out how many of the operations were
unaligned:
https://github.com/lathiat/ceph-tools/blob/master/fstore_op_latency.rb
You can just enable debug filestore = 10 dynamically on 1 OSD for about 5
minutes, turn it off, and process the log. And you could compare
before/after. I haven't written an equivalent tool for BlueStore
unfortunately if you are already in the modern world :) I also didnt' check
maybe debug osd or something also has the writes and offsets, so I could
write a generic tool to cover both cases, but also I have not done that.
Hope that helps.
Regards,
Trent
On Thu, 16 May 2019 at 14:52, Alexandre DERUMIER
wrote:
> Many thanks for the analysis !
>
>
> I'm going to test with 4K on heavy mssql database to see if I'm seeing
> improvement on ios/latency.
> I'll report results in this thread.
>
>
> - Mail original -
> De: "Trent Lloyd"
> À: "ceph-users"
> Envoyé: Vendredi 10 Mai 2019 09:59:39
> Objet: [ceph-users] Poor performance for 512b aligned "partial" writes
> from Windows guests in OpenStack + potential fix
>
> I recently was investigating a performance problem for a reasonably sized
> OpenStack deployment having around 220 OSDs (3.5" 7200 RPM SAS HDD) with
> NVMe Journals. The primary workload is Windows guests backed by Cinder RBD
> volumes.
> This specific deployment is Ceph Jewel (FileStore + SimpleMessenger) which
> while it is EOL, the issue is reproducible on current versions and also on
> BlueStore however for different reasons than FileStore.
>
> Generally the Ceph cluster was suffering from very poor outlier
> performance, the numbers change a little bit depending on the exact
> situation but roughly 80% of I/O was happening in a "reasonable" time of
> 0-200ms but 5-20% of I/O operations were taking excessively long anywhere
> from 500ms through to 10-20+ seconds. However the normal metrics for commit
> and apply latency were normal, and in fact, this latency was hard to spot
> in the performance metrics available in jewel.
>
> Previously I more simply considered FileStore to have the "commit" (to
> journal) stage where it was written to the journal and it is OK to return
> to the client and then the "apply" (to disk) stage where it was flushed to
> disk and confirmed so that the data could be purged from the journal.
> However there is really a third stage in the middle where FileStore submits
> the I/O to the operating system and this is done before the lock on the
> object is released. Until that succeeds another operation cannot write to
> the same object (generally being a 4MB area of the disk).
>
> I found that the fstore_op threads would get stuck for hundreds of MS or
> more inside of pwritev() which was blocking inside of the kernel. Normally
> we expect pwritev() to be buffered I/O into the page cache and return quite
> fast however in this case the kernel was in a few percent of cases blocking
> with the stack trace included at the end
Re: [ceph-users] Poor performance for 512b aligned "partial" writes from Windows guests in OpenStack + potential fix
Note that the issue I am talking about here is how a "Virtual" Ceph RBD disk is presented to a virtual guest, and specifically for Windows guests (Linux guests are not affected). I am not at all talking about how the physical disks are presented to Ceph itself (although Martin was, he wasn't clear whether changing these underlying physical disks to 4kn was for Ceph or other environments). I would not expect that having your underlying physical disk presented to Ceph itself as 512b/512e or 4kn to have a significant impact on performance for the reason that Linux systems generally send 4k-aligned I/O anyway (regardless of what the underlying disk is reporting for physical_block_size). There may be some exceptions to that, such as applications performing Direct I/O to the disk. If anyone knows otherwise, it would be great to hear specific details. Regards, Trent On Fri, May 10, 2019 at 4:40 PM Marc Roos wrote: > > Hmmm, so if I have (wd) drives that list this in smartctl output, I > should try and reformat them to 4k, which will give me better > performance? > > Sector Sizes: 512 bytes logical, 4096 bytes physical > > Do you have a link to this download? Can only find some .cz site with > the rpms. > > > -Original Message- > From: Martin Verges [mailto:martin.ver...@croit.io] > Sent: vrijdag 10 mei 2019 10:21 > To: Trent Lloyd > Cc: ceph-users > Subject: Re: [ceph-users] Poor performance for 512b aligned "partial" > writes from Windows guests in OpenStack + potential fix > > Hello Trent, > > many thanks for the insights. We always suggest to use 4kN over 512e > HDDs to our users. > > As we recently found out, is that WD Support offers a tool called HUGO > to reformat 512e to 4kN drives with "hugo format -m -n > max --fastformat -b 4096" in seconds. > Maybe that helps someone that has bought the wrong disk. > > -- > Martin Verges > Managing director > > Mobile: +49 174 9335695 > E-Mail: martin.ver...@croit.io > Chat: https://t.me/MartinVerges > > croit GmbH, Freseniusstr. 31h, 81247 Munich > CEO: Martin Verges - VAT-ID: DE310638492 Com. register: Amtsgericht > Munich HRB 231263 > > Web: https://croit.io > YouTube: https://goo.gl/PGE1Bx > > > > Am Fr., 10. Mai 2019 um 10:00 Uhr schrieb Trent Lloyd > : > > > I recently was investigating a performance problem for a > reasonably > sized OpenStack deployment having around 220 OSDs (3.5" 7200 RPM SAS > HDD) with NVMe Journals. The primary workload is Windows guests backed > by Cinder RBD volumes. > This specific deployment is Ceph Jewel (FileStore + > SimpleMessenger) which while it is EOL, the issue is reproducible on > current versions and also on BlueStore however for different reasons > than FileStore. > > > Generally the Ceph cluster was suffering from very poor outlier > performance, the numbers change a little bit depending on the exact > situation but roughly 80% of I/O was happening in a "reasonable" time of > 0-200ms but 5-20% of I/O operations were taking excessively long > anywhere from 500ms through to 10-20+ seconds. However the normal > metrics for commit and apply latency were normal, and in fact, this > latency was hard to spot in the performance metrics available in jewel. > > Previously I more simply considered FileStore to have the "commit" > (to journal) stage where it was written to the journal and it is OK to > return to the client and then the "apply" (to disk) stage where it was > flushed to disk and confirmed so that the data could be purged from the > journal. However there is really a third stage in the middle where > FileStore submits the I/O to the operating system and this is done > before the lock on the object is released. Until that succeeds another > operation cannot write to the same object (generally being a 4MB area of > the disk). > > I found that the fstore_op threads would get stuck for hundreds of > MS or more inside of pwritev() which was blocking inside of the kernel. > Normally we expect pwritev() to be buffered I/O into the page cache and > return quite fast however in this case the kernel was in a few percent > of cases blocking with the stack trace included at the end of the e-mail > [1]. My finding from that stack is that inside __block_write_begin_int > we see a call to out_of_line_wait_on_bit call which is really an inlined > call for wait_on_buffer which occurs in linux/fs/buffer.c in the section > around line 2000-2024 with the comment "If we issued read requests - let > them complete." > (https://github.com/torvalds/linux/blob/a2d635decbfa9c1e4ae15cb05b68b255 > 9f7f827c/fs/buffer.c#L2002 > <https://gith
[ceph-users] Poor performance for 512b aligned "partial" writes from Windows guests in OpenStack + potential fix
I recently was investigating a performance problem for a reasonably sized OpenStack deployment having around 220 OSDs (3.5" 7200 RPM SAS HDD) with NVMe Journals. The primary workload is Windows guests backed by Cinder RBD volumes. This specific deployment is Ceph Jewel (FileStore + SimpleMessenger) which while it is EOL, the issue is reproducible on current versions and also on BlueStore however for different reasons than FileStore. Generally the Ceph cluster was suffering from very poor outlier performance, the numbers change a little bit depending on the exact situation but roughly 80% of I/O was happening in a "reasonable" time of 0-200ms but 5-20% of I/O operations were taking excessively long anywhere from 500ms through to 10-20+ seconds. However the normal metrics for commit and apply latency were normal, and in fact, this latency was hard to spot in the performance metrics available in jewel. Previously I more simply considered FileStore to have the "commit" (to journal) stage where it was written to the journal and it is OK to return to the client and then the "apply" (to disk) stage where it was flushed to disk and confirmed so that the data could be purged from the journal. However there is really a third stage in the middle where FileStore submits the I/O to the operating system and this is done before the lock on the object is released. Until that succeeds another operation cannot write to the same object (generally being a 4MB area of the disk). I found that the fstore_op threads would get stuck for hundreds of MS or more inside of pwritev() which was blocking inside of the kernel. Normally we expect pwritev() to be buffered I/O into the page cache and return quite fast however in this case the kernel was in a few percent of cases blocking with the stack trace included at the end of the e-mail [1]. My finding from that stack is that inside __block_write_begin_int we see a call to out_of_line_wait_on_bit call which is really an inlined call for wait_on_buffer which occurs in linux/fs/buffer.c in the section around line 2000-2024 with the comment "If we issued read requests - let them complete." ( https://github.com/torvalds/linux/blob/a2d635decbfa9c1e4ae15cb05b68b2559f7f827c/fs/buffer.c#L2002 ) My interpretation of that code is that for Linux to store a write in the page cache, it has to have the entire 4K page as that is the granularity of which it tracks the dirty state and it needs the entire 4K page to later submit back to the disk. Since we wrote a part of the page, and the page wasn't already in the cache, it has to fetch the remainder of the page from the disk. When this happens, it blocks waiting for this read to complete before returning from the pwritev() call - hence our normally buffered write blocks. This holds up the tp_fstore_op thread, of which there are (by default) only 2-4 such threads trying to process several hundred operations per second. Additionally the size of the osd_op_queue is bounded, and operations do not clear out of this queue until the tp_fstore_op thread is done. Which ultimately means that not only are these partial writes delayed but it knocks on to delay other writes behind them because of the constrained thread pools. What was further confusing to this, is that I could easily reproduce this in a test deployment using an rbd benchmark that was only writing to a total disk size of 256MB which I would easily have expected to fit in the page cache: rbd create -p rbd --size=256M bench2 rbd bench-write -p rbd bench2 --io-size 512 --io-threads 256 --io-total 256M --io-pattern rand This is explained by the fact that on secondary OSDs (at least, there was some refactoring of fadvise which I have not fully understood as of yet), FileStore is using fadvise FADVISE_DONTNEED on the objects after write which causes the kernel to immediately discard them from the page cache without any regard to their statistics of being recently/frequently used. The motivation for this addition appears to be that on a secondary OSD we don't service reads (only writes) and so therefor we can optimize memory usage by throwing away this object and in theory leaving more room in the page cache for objects which we are primary for and expect to actually service reads from a client for. Unfortunately this behavior does not take into account partial writes, where we now pathologically throw away the cached copy instantly such that a write even 1 second later will have to fetch the page from disk again. I also found that this FADVISE_DONTNEED is issue not only during filestore sync but also by the WBThrottle - which as this cluster was quite busy was constantly flushing writes leading to the cache being discarded almost instantly. Changing filestore_fadvise to False on this cluster lead to a significant performance increase as it could now cache the pages in memory in many cases. The number of reads from disk was reduced from around 40/second to 2/second, and the number of slow writes
Re: [ceph-users] NFS interaction with RBD
Jens-Christian Fischer jens-christian.fischer@... writes: I think we (i.e. Christian) found the problem: We created a test VM with 9 mounted RBD volumes (no NFS server). As soon as he hit all disks, we started to experience these 120 second timeouts. We realized that the QEMU process on the hypervisor is opening a TCP connection to every OSD for every mounted volume - exceeding the 1024 FD limit. So no deep scrubbing etc, but simply to many connections… Have seen mention of similar from CERN in their presentations, found this post on a quick google.. might help? http://lists.ceph.com/pipermail/ceph-users-ceph.com/2013- December/026187.html Cheers, Trent ___ ceph-users mailing list ceph-users@lists.ceph.com http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
