[linux-lvm] very slow sequential writes on lvm raid1 (bitmap?)

Zdenek Kabelac zkabelac at redhat.com
Mon Nov 7 10:22:51 UTC 2016

Dne 7.11.2016 v 10:30 Alexander 'Leo' Bergolth napsal(a):
> Hi!
> I am experiencing a dramatic degradation of the sequential write speed
> on a raid1 LV that resides on two USB-3 connected harddisks (UAS
> enabled), compared to parallel access to both drives without raid or
> compared to MD raid:
> - parallel sequential writes LVs on both disks: 140 MB/s per disk
> - sequential write to MD raid1 without bitmap: 140 MB/s
> - sequential write to MD raid1 with bitmap: 48 MB/s
> - sequential write to LVM raid1: 17 MB/s !!
> According to the kernel messages, my 30 GB raid1-test-LV gets equipped
> with a 61440 bit write-intent bitmap (1 bit per 512 byte data?!) whereas
> a default MD raid1 bitmap only has 480 bit size. (1 bit per 64 MB).
> Maybe the dramatic slowdown is caused by this much too fine grained
> bitmap and its updates, which are random IO?
> Is there a way to configure the bitmap size?


Can you please provide some results with  '--regionsize'  changes ?
While   '64MB' is quite 'huge'  for resync I guess 'the current' default
picked region size is likely very very small in same cases.



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