Mike Snitzer wrote:
On Tue, May 06 2014 at 11:54am -0400,
Marc Caubet <mcaubet@pic.es> wrote:
Hi all,
I am trying to setup a storage pool with correct disk alignment and I hope
somebody can help me to understand some unclear parts to me when
configuring XFS over LVM2.
Actually we have few storage pools with the following settings each:
- LSI Controller with 3xRAID6
- Each RAID6 is configured with 10 data disks + 2 for double-parity.
- Each disk has a capacity of 4TB, 512e and physical sector size of 4K.
- 3x(10+2) configuration was considered in order to gain best performance
and data safety (less disks per RAID less probability of data corruption)
----
I have a similar setup and am almost certain I have 2 of them wrong as
shown below:
Model: LSI MR9280DE-8e (scsi)
Disk /dev/sda: 24.0TB
Sector size (logical/physical): 512B/512B
Partition Table: gpt_sync_mbr
Number Start End Size File system Name Flags
1 17.4kB 24.0TB 24.0TB home+shar lvm
Model: LSI MR9280DE-8e (scsi)
Disk /dev/sdb: 12.0TB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Number Start End Size File system Name Flags
1 1049kB 12.0TB 12.0TB Backups lvm
Model: DELL PERC 6/i (scsi)
Disk /dev/sdd: 7999GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt_sync_mbr
Number Start End Size File system Name Flags
1 17.4kB 7999GB 7999GB Media lvm
pvs says:
# pvs
PV VG Fmt Attr PSize PFree
/dev/sda1 HnS lvm2 a-- 21.83t 2.73t
/dev/sdb1 Backups lvm2 a-- 10.91t 3.15g
/dev/sdd1 Media lvm2 a-- 7.28t 0
-----
Notice how each of them are starting at some weird offset.
I thought I started /dev/sdb @ 1MB, which comes out to 1048576.. so sdb
might
be aligned on a sector boundary.....but has 6 data disks x 64K stripe, =
384K, which
doesn't divide into 1MB evenly.
/dev/sda has a strip-size of 768K, BUT since it is a RAID50 (3 RAID5's in a
RAID0 config), I can use 256K as a strip-size for writes, as a write of
any aligned 256K chunk will only affect 4 data disks (+ 1 parity).
And here is my first question: How can I check if the storage and the LV
are correctly aligned?
On the other hand, I have formatted XFS as follows:
mkfs.xfs -d su=256k,sw=10 -l size=128m,lazy-count=1 /dev/dcvg_a/dcpool
So my second question is, are the above 'su' and 'sw' parameters correct on
the current LV configuration? If not, which values should I have and why?
AFAIK su is the stripe size configured in the controller side, but in this
case we have a LV. Also, sw is the number of data disks in a RAID, but
again, we have a LV with 3 stripes, and I am not sure if the number of data
disks should be 30 instead.
Newer versions of mkfs.xfs _should_ pick up the hints exposed (as
minimum_io_size and optimal_io_size) by the striped LV.
----
But mkfs.xfs won't pick up the io_size optimal inside the LSI
controller.
That's underlying all of this. LVM didn't try to align space to even
some even amount
based on starting at 17.4k (i.e. would hve to round up to nearest 256 or
384 or 768K depending
on subsystem.
But if not you definitely don't want to be trying to pierce through the
striped LV config to establish settings of the underlying RAID6.
----
You have to.
Each
layer in the stack should respect the layer beneath it.
They don't. LV doesn't determine optimal start based on partition
start, so all of its
alignments are off.
My writes are noticeably slower than my reads sometimes by close to 10x
(5x in more general
case).
I hope to get another disk subsystem so I can dump those partitions and
align them, but
also, follow Stan Hoepper's advice from the xfs list -- go with a RAID
1+0... Then each
pair of RAID1 is independent of every other. The worst has to be that
768K. It triggers a bug
in the gnu database format which assumes the optimal I/O size will be a
power of 2
(which it is not, in my case).
_______________________________________________
linux-lvm mailing list
linux-lvm@redhat.com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
