very sub-optimal default readahead settings on device and unused readahead setting in LVM
Bug #129488 reported by
James Troup
This bug affects 7 people
Affects | Status | Importance | Assigned to | Milestone | |
---|---|---|---|---|---|
lvm2 (Ubuntu) |
Invalid
|
Medium
|
Unassigned | ||
Bug Description
Binary package hint: lvm2
When you create an LV, the device ends up with a readahead of 256 512-byte sectors. A normal (non-LVM) device's readahead appears to default to 8192 512-byte sectors. This makes LVM FSs benchmark (and perform) very badly (our read rate dropped from 320M/s to 90M/s).
To make matters worse, LVM has an internal 'read ahead sectors' variable which is apparently unused, but still displayed by e.g. lvdisplay which just adds to the confusion.
This is apparently a known issue upstream:
description: | updated |
description: | updated |
Changed in lvm2 (Ubuntu): | |
status: | Confirmed → Invalid |
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From irc, work-around after boot is:
blockdev --setra 8192 /dev/vg/lv
We need to find the right place to fix this by default. Kernel patch needed?