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Re: [linux-lvm] (LONG) Delay when writing to LVM after boot



Likely a question for ext4 forum - I guess FS is reading[1] and caching[2] additional metadata to find suitable block.

[1] explains the delay on first write
[2] explains no delay on subsequent writes

-- Marian

On 04/19/2013 03:16 AM, Ken Bass wrote:
I have one LVG with one LV consisting of several physical drives, ranging
from 500G to 3T, with a total size of 6.5T, formatted with ext4.

After I boot the system, when anything writes a file to that LV, the write
hangs for 1 minute or more, then continues at full speed with no errors.
Some apps, though can't handle the delay, and report that the write fails.

This does not occur with a small file ( < 50M or so ). But the first time a
larger file is written, then the delay. And this delay occurs only one time.

I have noticed something similar in the past with smaller LVs ( < 1T ), but
the delay was insignificant then.  But as I grew this LV, the delay
increased.

I can run e2fsck on the LV, and even optimize directories (-D option), with
no delay occurring.

I have tried googling, but have not found any mention of something like
this (maybe not using the right key words?). So, does this sound familiar
to any LVM experts out there? Any suggestions?

FWIW: I'm currently running Fedora 17 (64 bit), kernel
3.8.4-102.fc17.x86_64 (although I have seen this problem on 32 bit
kernels/machines as well). I have 4 physical drives (500G, 1T, 1.5T, 3T),
and non-raid configured.

Some other info:

[root elmer ken]# lvmdiskscan
   /dev/vg_elmer/lv_swap [       3.62 GiB]
   /dev/sda1             [     500.00 MiB]
   /dev/vg_elmer/lv_root [      23.81 GiB]
   /dev/sda2             [      27.47 GiB] LVM physical volume
   /dev/VG_NAS/LV_NAS    [       5.46 TiB]
   /dev/sdb1             [       2.73 TiB] LVM physical volume
   /dev/sdc1             [     465.76 GiB] LVM physical volume
   /dev/sdd1             [     931.51 GiB] LVM physical volume
   /dev/sde1             [       1.36 TiB] LVM physical volume
   3 disks
   1 partition
   0 LVM physical volume whole disks
   5 LVM physical volumes
[root elmer ken]# lvdisplay
   --- Logical volume ---
   LV Path                /dev/VG_NAS/LV_NAS
   LV Name                LV_NAS
   VG Name                VG_NAS
   LV UUID                IO93PY-kFF2-19u7-33lX-m7Mg-8BZC-q12ttn
   LV Write Access        read/write
   LV Creation host, time ,
   LV Status              available
   # open                 1
   LV Size                5.46 TiB
   Current LE             1430794
   Segments               5
   Allocation             inherit
   Read ahead sectors     auto
   - currently set to     256
   Block device           253:2
[root elmer ken]#

(note: sda is separate physical drive with system/kernel only - vg_elmer-*)

T IA

ken



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