Minimum inode cache size? (was: Slow file operations on file server with 30 TB hardware RAID and 100 TB software RAID)

From: Paul Menzel
Date: Thu Aug 26 2021 - 06:41:34 EST

Dear Linux folks,


Am 20.08.21 um 16:39 schrieb Paul Menzel:

Am 20.08.21 um 16:31 schrieb Paul Menzel:

Short problem statement: Sometimes changing into a directory on a file server wit 30 TB hardware RAID and 100 TB software RAID both formatted with XFS takes several seconds.


On a Dell PowerEdge T630 with two Xeon CPU E5-2603 v4 @ 1.70GHz and 96 GB RAM a 30 TB hardware RAID is served by the hardware RAID controller and a 100 TB MDRAID software RAID connected to a Microchip 1100-8e both formatted using XFS. Currently, Linux 5.4.39 runs on it.

```
$ more /proc/version
Linux version 5.4.39.mx64.334 (root@xxxxxxxxxxxxxxxxx) (gcc version 7.5.0 (GCC)) #1 SMP Thu May 7 14:27:50 CEST 2020
$ dmesg | grep megar
[   10.322823] megaraid cmm: 2.20.2.7 (Release Date: Sun Jul 16 00:01:03 EST 2006)
[   10.331910] megaraid: 2.20.5.1 (Release Date: Thu Nov 16 15:32:35 EST 2006)
[   10.345055] megaraid_sas 0000:03:00.0: BAR:0x1  BAR's base_addr(phys):0x0000000092100000  mapped virt_addr:0x0000000059ea5995
[   10.345057] megaraid_sas 0000:03:00.0: FW now in Ready state
[   10.351868] megaraid_sas 0000:03:00.0: 63 bit DMA mask and 32 bit consistent mask
[   10.361655] megaraid_sas 0000:03:00.0: firmware supports msix    : (96)
[   10.369433] megaraid_sas 0000:03:00.0: requested/available msix 13/13
[   10.377113] megaraid_sas 0000:03:00.0: current msix/online cpus    : (13/12)
[   10.385190] megaraid_sas 0000:03:00.0: RDPQ mode    : (disabled)
[   10.392092] megaraid_sas 0000:03:00.0: Current firmware supports maximum commands: 928     LDIO threshold: 0
[   10.403895] megaraid_sas 0000:03:00.0: Configured max firmware commands: 927
[   10.416840] megaraid_sas 0000:03:00.0: Performance mode :Latency
[   10.424029] megaraid_sas 0000:03:00.0: FW supports sync cache    : No
[   10.431417] megaraid_sas 0000:03:00.0: megasas_disable_intr_fusion is called outbound_intr_mask:0x40000009
[   10.486158] megaraid_sas 0000:03:00.0: FW provided supportMaxExtLDs: 1    max_lds: 64
[   10.495502] megaraid_sas 0000:03:00.0: controller type    : MR(2048MB)
[   10.502988] megaraid_sas 0000:03:00.0: Online Controller Reset(OCR)    : Enabled
[   10.511445] megaraid_sas 0000:03:00.0: Secure JBOD support    : No
[   10.518543] megaraid_sas 0000:03:00.0: NVMe passthru support    : No
[   10.525834] megaraid_sas 0000:03:00.0: FW provided TM TaskAbort/Reset timeout: 0 secs/0 secs
[   10.536251] megaraid_sas 0000:03:00.0: JBOD sequence map support    : No
[   10.543931] megaraid_sas 0000:03:00.0: PCI Lane Margining support    : No
[   10.574406] megaraid_sas 0000:03:00.0: megasas_enable_intr_fusion is called outbound_intr_mask:0x40000000
[   10.585995] megaraid_sas 0000:03:00.0: INIT adapter done
[   10.592409] megaraid_sas 0000:03:00.0: JBOD sequence map is disabled megasas_setup_jbod_map 5660
[   10.603273] megaraid_sas 0000:03:00.0: pci id        : (0x1000)/(0x005d)/(0x1028)/(0x1f42)
[   10.612815] megaraid_sas 0000:03:00.0: unevenspan support    : yes
[   10.619919] megaraid_sas 0000:03:00.0: firmware crash dump    : no
[   10.627013] megaraid_sas 0000:03:00.0: JBOD sequence map    : disabled
$ dmesg | grep 1100-8e
[   25.853170] smartpqi 0000:84:00.0: added 11:2:0:0 0000000000000000 RAID              Adaptec  1100-8e
[   25.867069] scsi 11:2:0:0: RAID              Adaptec  1100-8e  2.93 PQ: 0 ANSI: 5
$ xfs_info /dev/sdc
meta-data=/dev/sdc               isize=512    agcount=28, agsize=268435455 blks
          =                       sectsz=512   attr=2, projid32bit=1
          =                       crc=1        finobt=1, sparse=0, rmapbt=0
          =                       reflink=0
data     =                       bsize=4096   blocks=7323648000, imaxpct=5
          =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
log      =internal log           bsize=4096   blocks=521728, version=2
          =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
$ xfs_info /dev/md0
meta-data=/dev/md0               isize=512    agcount=102, agsize=268435328 blks
          =                       sectsz=4096  attr=2, projid32bit=1
          =                       crc=1        finobt=1, sparse=0, rmapbt=0
          =                       reflink=0
data     =                       bsize=4096   blocks=27348633088, imaxpct=1
          =                       sunit=128    swidth=1792 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
log      =internal log           bsize=4096   blocks=521728, version=2
          =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
$ df -i /dev/sdc
Filesystem         Inodes   IUsed      IFree IUse% Mounted on
/dev/sdc       2929459200 4985849 2924473351    1% /home/pmenzel
$ df -i /dev/md0
Filesystem         Inodes   IUsed      IFree IUse% Mounted on
/dev/md0       2187890624 5331603 2182559021    1% /jbod/M8015
```

After not using a directory for a while (over 24 hours), changing into it (locally) takes over five seconds or doing some git operations. For example the Linux kernel source git tree located in my home directory. (My shell has some git integration showing the branch name in the prompt (`/usr/share/git-contrib/completion/git-prompt.sh`.) Once in that directory, everything reacts instantly again. When waiting the Linux pressure stall information (PSI) shows IO resource contention.

Before:

     $ grep -R . /proc/pressure/
     /proc/pressure/io:some avg10=0.40 avg60=0.10 avg300=0.10 total=48330841502
     /proc/pressure/io:full avg10=0.40 avg60=0.10 avg300=0.10 total=48067233340
     /proc/pressure/cpu:some avg10=0.00 avg60=0.00 avg300=0.00 total=755842910
     /proc/pressure/memory:some avg10=0.00 avg60=0.00 avg300=0.00 total=2530206336
     /proc/pressure/memory:full avg10=0.00 avg60=0.00 avg300=0.00 total=2318140732

During `git log stable/linux-5.10.y`:

     $ grep -R . /proc/pressure/
     /proc/pressure/io:some avg10=26.20 avg60=9.72 avg300=2.37 total=48337351849
     /proc/pressure/io:full avg10=26.20 avg60=9.72 avg300=2.37 total=48073742033
     /proc/pressure/cpu:some avg10=0.00 avg60=0.00 avg300=0.00 total=755843898
     /proc/pressure/memory:some avg10=0.00 avg60=0.00 avg300=0.00 total=2530209046
     /proc/pressure/memory:full avg10=0.00 avg60=0.00 avg300=0.00 total=2318143440

The current explanation is, that over night several maintenance scripts like backup/mirroring and accounting scripts are run, which touch all files on the devices. Additionally sometimes other users run cluster jobs with millions of files on the software RAID. Such things invalidate the inode cache, and “my” are thrown out. When I use it afterward it’s slow in the beginning. There is still free memory during these times according to `top`.

    $ free -h
                  total        used        free      shared  buff/cache available
    Mem:            94G        8.3G        5.3G        2.3M         80G       83G
    Swap:            0B          0B          0B

Does that sound reasonable with ten million inodes? Is that easily verifiable?

If an inode consume 512 bytes with ten million inodes, that would be around 500 MB, which should easily fit into the cache, so it does not need to be invalidated?

Something is wrong with that calculation, and the cache size is much bigger.

Looking into `/proc/slabinfo` and XFS’ runtime/internal statistics [1], it turns out that the inode cache is likely the problem.

XFS’ internal stats show that only one third of the inodes requests are answered from cache.

$ grep ^ig /sys/fs/xfs/stats/stats
ig 1791207386 647353522 20111 1143854223 394 1142080045 10683174

During the problematic time, the SLAB size is around 4 GB and, according to slabinfo, the inode cache only has around 200.000 (sometimes even as low as 50.000).

$ sudo grep inode /proc/slabinfo
nfs_inode_cache 16 24 1064 3 1 : tunables 24 12 8 : slabdata 8 8 0
rpc_inode_cache 94 138 640 6 1 : tunables 54 27 8 : slabdata 23 23 0
mqueue_inode_cache 1 4 896 4 1 : tunables 54 27 8 : slabdata 1 1 0
xfs_inode 1693683 1722284 960 4 1 : tunables 54 27 8 : slabdata 430571 430571 0
ext2_inode_cache 0 0 768 5 1 : tunables 54 27 8 : slabdata 0 0 0
reiser_inode_cache 0 0 760 5 1 : tunables 54 27 8 : slabdata 0 0 0
hugetlbfs_inode_cache 2 12 608 6 1 : tunables 54 27 8 : slabdata 2 2 0
sock_inode_cache 346 670 768 5 1 : tunables 54 27 8 : slabdata 134 134 0
proc_inode_cache 121 288 656 6 1 : tunables 54 27 8 : slabdata 48 48 0
shmem_inode_cache 2249 2827 696 11 2 : tunables 54 27 8 : slabdata 257 257 0
inode_cache 209098 209482 584 7 1 : tunables 54 27 8 : slabdata 29926 29926 0

(What is the difference between `xfs_inode` and `inode_cache`?)

Then going through all the files with `find -ls`, the inode cache grows to four to five million and the SLAB size grows to around 8 GB. Over night it shrinks back to the numbers above and the page cache grows back.

In the discussions [2], adji`vfs_cache_pressure` is recommended, but – besides setting it to 0 – it only seems to delay the shrinking of the cache. (As it’s an integer 1 is the lowest non-zero (positive) number, which would delay it by a factor of 100.

Is there a way to specify the minimum numbers of entries in the inode cache, or a minimum SLAB size up to that the caches should not be decreased?


Kind regards,

Paul


[1]: https://xfs.org/index.php/Runtime_Stats#ig
[2]: https://linux-xfs.oss.sgi.narkive.com/qa0AYeBS/improving-xfs-file-system-inode-performance
"Improving XFS file system inode performance" from 2010