zfs discuss - Nov 2009 - Using local disk for cache on an iSCSI zvol...

I''m just wondering if anyone has tried this, and what the performance 
has been like.

Scenario:

I''ve got a bunch of v20z machines, with 2 disks.  One has the OS on it,
and the other is free. As these are disposable client machines, I''m not
going to mirror the OS disk.

I have a disk server with a striped mirror zpool, carved into a bunch of 
zvols, each exported via iSCSI.  I mount one of these iSCSI zvols on 
each v20z client.

What I''m wondering is this:   if, on the client v20z, I make a zpool of
the iSCSI volume, what kind of performance impact would I have if I add 
the second local disk as either (or perhaps both) a ZIL or L2ARC  
caching device?  These are speedy 15k SCSI drives. 

I realize that if I have to migrate the iSCSI zvol off the original 
client, I lose access to my cache device, but that''s OK.  I''m
just
thinking in terms of performance.  I guess what it boils down to is what 
is the access time/throughput of a single local 15k SCSI drive vs a GigE 
iSCSI volume?
 

--
Erik Trimble
Java System Support
Mailstop:  usca22-123
Phone:  x17195
Santa Clara, CA

Erik Trimble wrote:
> I''m just wondering if anyone has tried this, and what the
performance
> has been like.
> 
> Scenario:
> 
> I''ve got a bunch of v20z machines, with 2 disks.  One has the OS
on it,
> and the other is free. As these are disposable client machines,
I''m not
> going to mirror the OS disk.
> 
> I have a disk server with a striped mirror zpool, carved into a bunch of 
> zvols, each exported via iSCSI.  I mount one of these iSCSI zvols on 
> each v20z client.
> 
> What I''m wondering is this:   if, on the client v20z, I make a
zpool of
> the iSCSI volume, what kind of performance impact would I have if I add 
> the second local disk as either (or perhaps both) a ZIL or L2ARC  
I wouldn''t recommend using it as a ZIL because then you don''t
get to
import that pool on another physical machine.  There could be needed 
data on the ZIL.
> caching device?  These are speedy 15k SCSI drives.
> I realize that if I have to migrate the iSCSI zvol off the original 
> client, I lose access to my cache device, but that''s OK.
Right, because the cache isn''t even currently persistent accross a 
reboot (or pool export) anyway.

 >  I''m just
> thinking in terms of performance.  I guess what it boils down to is what 
> is the access time/throughput of a single local 15k SCSI drive vs a GigE 
> iSCSI volume?
For the L2ARC it is worth at try - what have you got to loose since you 
can remove the cache device really easily.

-- 
Darren J Moffat