Il 15-01-2020 17:39 Darrick J. Wong ha scritto:
extszinherit > 0 disables delayed allocation, which means that (in your case above) if you wrote 1G to a file (using the pagecache) you'd get 8192x 128K calls to the allocator instead of making a single 1G allocation during writeback.
Thanks for the valuable information, I did not know that specific interaction between extsize and delalloc.
If you have a lot of memory (or a high vmm dirty ratio) then you want delalloc over extsize. Most of the time you want delalloc, frankly.
Let me briefly describe the expected workload: thinly provisioned virtual image storage. The problem with "plain" sparse file (ie: without extsize hint) is that, after some time, the underlying vdisk file will be very fragmented: consecutive physical blocks will be assigned to very different logical blocks, leading to sub-par performance when reading back the whole file (eg: for backup purpose).
I can easily simulate a worst-case scenario with fio, issuing random write to a pre-created sparse file. While the random writes complete very fast (because they are more-or-less sequentially written inside the sparse file), reading back that file will have very low performance: 10 MB/s vs 600+ MB/s for a preallocated file.
Using a 128k extsize brings sequential read to ~100 MB/s (which is reasonable on that old hardware), and a 16M extsize is in the range of 500+ MB/s.
Given that use case, do you suggest sticking with delalloc or setting an appropriate extsize?
Thanks. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx GPG public key ID: FF5F32A8
