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Re: maxpct option for small xfs filesystems

To: Eric Sandeen <sandeen@xxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: maxpct option for small xfs filesystems
From: Alexander Tsvetkov <alexander.tsvetkov@xxxxxxxxxx>
Date: Wed, 28 Jan 2015 13:41:48 +0300
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <54C7BD60.5000104@xxxxxxxxxxx>
References: <54C667F3.8040303@xxxxxxxxxx> <20150126223715.GA7621@dastard> <54C7BB78.4060203@xxxxxxxxxx> <54C7BD60.5000104@xxxxxxxxxxx>
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On 01/27/2015 07:31 PM, Eric Sandeen wrote:
On 1/27/15 10:23 AM, Alexander Tsvetkov wrote:

...

I have not the same results,  just installed 3.19-rc6 and repeated the test.,
df -i reports 640 inodes for filesystem, but actually created 40512 files:

[root@fedora ~]# mkfs.xfs -f -d size=16m -i maxpct=1 /dev/sdb2
meta-data=/dev/sdb2              isize=256    agcount=1, agsize=4096 blks
          =                       sectsz=512   attr=2, projid32bit=1
          =                       crc=0        finobt=0
data     =                       bsize=4096   blocks=4096, imaxpct=1
          =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
log      =internal log           bsize=4096   blocks=853, version=2
          =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
[root@fedora ~]# mount /dev/sdb2 /mnt/scratch/
fill with files until enospc...
[root@fedora ~]# df -i /mnt/scratch/
Filesystem     Inodes IUsed IFree IUse% Mounted on
/dev/sdb2         640   640     0  100% /mnt/scratch
[root@fedora ~]# df -Th /mnt/scratch/
Filesystem     Type  Size  Used Avail Use% Mounted on
/dev/sdb2      xfs    13M   13M  156K  99% /mnt/scratch
[root@fedora ~]# umount /mnt/scratch
[root@fedora ~]# xfs_db -c "blockget -n" -c "ncheck" /dev/sdb2 | wc -l
40512
and what does df -i say after remount?
Nothing changed after remount:

[root@fedora ~]# xfs_db -c "blockget -n" -c "ncheck" /dev/sdb2 | wc -l
40512
[root@fedora ~]# mount /dev/sdb2 /mnt/scratch/
[root@fedora ~]# xfs_info /mnt/scratch/
meta-data=/dev/sdb2 isize=256 agcount=1, agsize=4096 blks
                 =                sectsz=512   attr=2, projid32bit=1
                 =                crc=0 finobt=0
data          =                bsize=4096   blocks=4096, imaxpct=1
                 =                sunit=0      swidth=0 blks
naming     =version 2 bsize=4096   ascii-ci=0 ftype=0
log            =internal    bsize=4096   blocks=853, version=2
                 =                sectsz=512   sunit=0 blks, lazy-count=1
realtime    =none        extsz=4096   blocks=0, rtextents=0
[root@fedora ~]# df -i /mnt/scratch/
Filesystem     Inodes IUsed IFree IUse% Mounted on
/dev/sdb2         640   640     0  100% /mnt/scratch
[root@fedora ~]# umount /mnt/scratch
[root@fedora ~]# xfs_db -c "blockget -n" -c "ncheck" /dev/sdb2 | wc -l
40512
[root@fedora ~]# mount /dev/sdb2 /mnt/scratch/
[root@fedora ~]# df -i /mnt/scratch/
Filesystem     Inodes IUsed IFree IUse% Mounted on
/dev/sdb2         640   640     0  100% /mnt/scratch
[root@fedora ~]# mount -o remount /dev/sdb2 /mnt/scratch/
[root@fedora ~]# df -i /mnt/scratch/
Filesystem     Inodes IUsed IFree IUse% Mounted on
/dev/sdb2         640   640     0  100% /mnt/scratch
[root@fedora ~]# umount /mnt/scratch
[root@fedora ~]# xfs_db -c "blockget -n" -c "ncheck" /dev/sdb2 | wc -l
40512


This is actually a problem with the lazy superblock counters I've run into 
before,
but haven't yet fixed.  This kind of workload is such that it never trips the
runtime rebalancing.

Looking into ncheck output there are 40512 pairs reported in the output each 
with own unique
inode number. ncheck doesn't report inodes count by definition, but what does 
these
40512 reported inode numbers mean if only actually 640 inodes were allocated? 
From another hand
each new file should have associated meta-data in the corresponding allocated 
inode structure, so for
40512 newly created files I expect the same count of allocated inodes, is it 
correct?
Recheck df -i after remount, I think you will see many more than 640.
Do you mean that ncheck reports right number of allocated inodes instead of df -i? If counters are incorrect then the maxpct limit is not working as well, because there are much more of filesystem space allocated for inodes then defined limit 1%.

What is expected behaviour in this case when it's required for fs to allocate new inodes starting to exceed defined maxpct percentage? Which error is expected to be returned to user, enospc or probably just some warning?



-Eric
Thanks,
Alexander Tsvetkov

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