Re: [linux-lvm] Paralell IO performance with LVM and XFS

[Stephen Lord <lord@sgi.com>]

Miguel Angel de Vega wrote:

>Hi!
>
>I've a Dual P2 450 with a Disk Array with 4 scsi disks with XFS.
>With one single disc I've 10 Mb/s reading and writing, so I think that if I 
>stripe all disc of the array, it can give me 4x10Mb = 40Mb/s more or less, 
>but I only gets 10 Mg/s!!  
>These disks usually gives up to 30 Mg/s in SGI machines and other single scsi 
>disks in the Dual P2 (with the same controller and XFS) gives 20Mg/s, so i 
>think that the problem must be LVM (the options maybe??? bad stripped 
>options??). 
>

Because LVM has difficulties dealing with the differing request sizes issued
by XFS, we feed it requests in small chunks (which means lots of buffer 
heads).
XFS has some 512 byte chunks of metadata, it tends to want to do metadata
I/O in sizes of 512 bytes, 4K, 8K and various random sizes upto 32K for 
the log writes.

Since request mapping in LVM basically consists of taking an existing 
buffer head and
giving it a different address xfs takes a very conservative approach. 
Striping with the
raid code should give you considerably better performance.

Now if someone would go off and make lvm work with the bio interface in 
2.5 things
might go pretty fast.

Steve

>
>
>I show how mount all the Logical Volumen:
>
>partionated all discs like 8e
>#pvcreate /dev/sda1 ...
>#vgcreate test /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
>#lvcreate -n stripped -i 4 -I 64 test -L 30000M
>(I tryed with -I 128, 256 and 512 but not big differences)
>
>#mkfs -t xfs /dev/test/stripped  (only supports 4kb of block size)
>
>what can be the problem??
>thanks
>
>Miguel Angel de Vega fernandez
>SGO
>Madrid - Spain
>
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