read-ahead for md/lvm combination

read-ahead for md/lvm combination

[Daniel Pocock]

While I was adding the SCTERC settings to my rc.local, I had a look over
/sbin/blockdev --setra ... settings

I do things like this, for example:

/sbin/blockdev --setra 8192 /dev/mapper/myvg-large_media_files

What about
- the devices /dev/sd[ab]2
- the md device, /dev/md2  (which is the PV for LVM) ?

Would it be correct to leave /dev/sd[ab]2 and /dev/md2 at defaults, and
only specify larger sizes for LVs that need it?  Or should I even make
/dev/sd[ab]2 and /dev/md2 smaller than the default value (256)?


Those disks are SATA - but I'm also planning to add in some SSDs and
RAID1 them with md - they will hold things like source code and home
directories (to make compiling and NFS home directories faster).  Can
anyone comment on the read ahead config or any other aspect of such a plan?

Re: read-ahead for md/lvm combination

[NeilBrown]

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On Fri, 11 May 2012 14:55:34 +0000 Daniel Pocock <daniel@pocock.com.au> wrote:

> 
> 
> While I was adding the SCTERC settings to my rc.local, I had a look over
> /sbin/blockdev --setra ... settings
> 
> I do things like this, for example:
> 
> /sbin/blockdev --setra 8192 /dev/mapper/myvg-large_media_files
> 
> What about
> - the devices /dev/sd[ab]2
> - the md device, /dev/md2  (which is the PV for LVM) ?
> 
> Would it be correct to leave /dev/sd[ab]2 and /dev/md2 at defaults, and
> only specify larger sizes for LVs that need it?  Or should I even make
> /dev/sd[ab]2 and /dev/md2 smaller than the default value (256)?

The read-ahead setting is interpreted by the pagecache.
So it only makes sense to set in on a device which is directly mounted or
accessed via /dev/XXX.

On other devices - such as the media device beneath and md array - you can
set the read-ahead to anything or nothing and it will have no effect at all.

NeilBrown

> 
> 
> Those disks are SATA - but I'm also planning to add in some SSDs and
> RAID1 them with md - they will hold things like source code and home
> directories (to make compiling and NFS home directories faster).  Can
> anyone comment on the read ahead config or any other aspect of such a plan?
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