Re: BTRFS as a GlusterFS storage back-end, and what I've learned from using it as such.

[Sargun Dhillon <sargun@sargun.me>]

Not to change the topic too much but, is there a suite of tracing
scripts that one can attach to their BtrFS installation to gather
metrics about tree locking performance? We see an awful lot of
machines with a task waiting on btrfs_tree_lock, and a bunch of other
tasks that are also in disk sleep waiting on BtrFS. We also see a
bunch of hung timeouts around btrfs_destroy_inode -- We're running
Kernel 4.8, so we can pretty easily plug in BPF based probes into the
kernel to get this information, and aggregate it.

Rather than doing this work ourselves, I'm wondering if anyone else
has a good set of tools to collect perf data about BtrFS performance
and lock contention?

On Tue, Apr 11, 2017 at 10:49 PM, Qu Wenruo <quwenruo@cn.fujitsu.com> wrote:
>
>
> At 04/11/2017 11:40 PM, Austin S. Hemmelgarn wrote:
>>
>> About a year ago now, I decided to set up a small storage cluster to store
>> backups (and partially replace Dropbox for my usage, but that's a separate
>> story).  I ended up using GlusterFS as the clustering software itself, and
>> BTRFS as the back-end storage.
>>
>> GlusterFS itself is actually a pretty easy workload as far as cluster
>> software goes.  It does some processing prior to actually storing the data
>> (a significant amount in fact), but the actual on-device storage on any
>> given node is pretty simple.  You have the full directory structure for the
>> whole volume, and whatever files happen to be on that node are located
>> within that tree exactly like they are in the GlusterFS volume. Beyond the
>> basic data, gluster only stores 2-4 xattrs per-file (which are used to track
>> synchronization, and also for it's internal data scrubbing), and a directory
>> called .glusterfs in the top of the back-end storage location for the volume
>> which contains the data required to figure out which node a file is on.
>> Overall, the access patterns mostly mirror whatever is using the Gluster
>> volume, or are reduced to slow streaming writes (when writing files and the
>> back-end nodes are computationally limited instead of I/O limited), with the
>> addition of some serious metadata operations in the .glusterfs directory
>> (lots of stat calls there, together with large numbers of small files).
>
>
> Any real world experience is welcomed to share.
>
>>
>> As far as overall performance, BTRFS is actually on par for this usage
>> with both ext4 and XFS (at least, on my hardware it is), and I actually see
>> more SSD friendly access patterns when using BTRFS in this case than any
>> other FS I tried.
>
>
> We also find that, for pure buffered read/write, btrfs is no worse than
> traditional fs.
>
> In our PostgreSQL test, btrfs can even get a little better performance than
> ext4/xfs when handling DB files.
>
> But if using btrfs for PostgreSQL Write Ahead Log (WAL), then it's
> completely another thing.
> Btrfs falls far behind ext4/xfs on HDD, only half of the TPC performance for
> low concurrency load.
>
> Due to btrfs CoW, btrfs causes extra IO for fsync.
> For example, if only to fsync 4K data, btrfs can cause 64K metadata write
> for default mkfs options.
> (One tree block for log root tree, one tree block for log tree, multiple by
> 2 for default DUP profile)
>
>>
>> After some serious experimentation with various configurations for this
>> during the past few months, I've noticed a handful of other things:
>>
>> 1. The 'ssd' mount option does not actually improve performance on these
>> SSD's.  To a certain extent, this actually surprised me at first, but having
>> seen Hans' e-mail and what he found about this option, it actually makes
>> sense, since erase-blocks on these devices are 4MB, not 2MB, and the drives
>> have a very good FTL (so they will aggregate all the little writes
>> properly).
>>
>> Given this, I'm beginning to wonder if it actually makes sense to not
>> automatically enable this on mount when dealing with certain types of
>> storage (for example, most SATA and SAS SSD's have reasonably good FTL's, so
>> I would expect them to have similar behavior).  Extrapolating further, it
>> might instead make sense to just never automatically enable this, and expose
>> the value this option is manipulating as a mount option as there are other
>> circumstances where setting specific values could improve performance (for
>> example, if you're on hardware RAID6, setting this to the stripe size would
>> probably improve performance on many cheaper controllers).
>>
>> 2. Up to a certain point, running a single larger BTRFS volume with
>> multiple sub-volumes is more computationally efficient than running multiple
>> smaller BTRFS volumes.  More specifically, there is lower load on the system
>> and lower CPU utilization by BTRFS itself without much noticeable difference
>> in performance (in my tests it was about 0.5-1% performance difference,
>> YMMV).  To a certain extent this makes some sense, but the turnover point
>> was actually a lot higher than I expected (with this workload, the turnover
>> point was around half a terabyte).
>
>
> This seems to be related to tree locking overhead.
>
> The most obvious solution is just as you stated, use many small subvolumes
> other than one large subvolume.
>
> Another less obvious solution is to reduce tree block size at mkfs time.
>
> This Btrfs is not that good at handling metadata workload, limited by both
> the overhead of mandatory metadata CoW and current tree lock algorithm.
>
>>
>> I believe this to be a side-effect of how we use per-filesystem
>> worker-pools.  In essence, we can schedule parallel access better when it's
>> all through the same worker pool than we can when using multiple worker
>> pools.  Having realized this, I think it might be interesting to see if
>> using a worker-pool per physical device (or at least what the system sees as
>> a physical device) might make more sense in terms of performance than our
>> current method of using a pool per-filesystem.
>>
>> 3. On these SSD's, running a single partition in dup mode is actually
>> marginally more efficient than running 2 partitions in raid1 mode.  I was
>> actually somewhat surprised by this, and I haven't been able to find a clear
>> explanation as to why (I suspect caching may have something to do with it,
>> but I'm not 100% certain about that),  but some limited testing with other
>> SSD's seems to indicate that it's the case for most SSD's, with the
>> difference being smaller on smaller and faster devices. On a traditional
>> hard disk, it's significantly more efficient, but that's generally to be
>> expected.
>>
>> 4. Depending on other factors, compression can actually slow you down
>> pretty significantly.  In the particular case I saw this happen (all cores
>> completely utilized by userspace software), LZO compression actually caused
>> around 5-10% performance degradation compared to no compression.  This is
>> somewhat obvious once it's explained, but it's not exactly intuitive  and as
>> such it's probably worth documenting in the man pages that compression won't
>> always make things better.  I may send a patch to add this at some point in
>> the near future.
>
>
> This seems interesting.
> Maybe it's CPU limiting the performance?
>
> Thanks,
> Qu
>
>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>>
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html