On 13/01/20 13:21, Gionatan Danti wrote:
On 13/01/20 12:43, Carlos Maiolino wrote:
I should have mentioned it, my apologies.
'extsize' argument for mkfs.xfs will set the size of the blocks in the RT
section.
Although, the 'extsize' command in xfs_io, will set the extent size
hints on any
file of any xfs filesystem (or filesystem supporting FS_IOC_FSSETXATTR).
Notice you can use xfs_io extsize to set the extent size hint to a
directory,
and all files under the directory will inherit the same extent hint.
My bad, I forgot about xfs_io.
Thanks for the detailed explanation.
Well, I did some test with a reflinked file and I must say I am
impressed on how well XFS handles small rewrites (for example 4K).
From my understanding, by mapping at 4K granularity but allocating at
128K, it avoid most read/write amplification *and* keep low
fragmentation. After "speculative_cow_prealloc_lifetime" it reclaim the
allocated but unused space, bringing back any available free space to
the filesystem. Is this understanding correct?
I have a question: how can I see the allocated-but-unused cow extents?
For example, giving the following files:
[root@neutron xfs]# stat test.img copy.img
File: test.img
Size: 1073741824 Blocks: 2097400 IO Block: 4096 regular file
Device: 810h/2064d Inode: 131 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Context: unconfined_u:object_r:unlabeled_t:s0
Access: 2020-01-13 15:40:50.280711297 +0100
Modify: 2020-01-13 16:21:55.564726283 +0100
Change: 2020-01-13 16:21:55.564726283 +0100
Birth: -
File: copy.img
Size: 1073741824 Blocks: 2097152 IO Block: 4096 regular file
Device: 810h/2064d Inode: 132 Links: 1
Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
Context: unconfined_u:object_r:unlabeled_t:s0
Access: 2020-01-13 15:40:50.280711297 +0100
Modify: 2020-01-13 15:40:57.828552412 +0100
Change: 2020-01-13 15:41:48.190492279 +0100
Birth: -
I can clearly see that test.img has an additional 124K allocated after a
4K rewrite. This matches my expectation: a 4K rewrite really allocates a
128K blocks, leading to 124K of temporarily "wasted" space.
But both "filefrag -v" and "xfs_bmap -vep" show only the used space as
seen by an userspace application (ie: 262144 blocks of 4096 bytes =
1073741824 bytes).
How can I check the total allocated space as reported by stat?
Thanks.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
