[CentOS] OT: Caching synchronous writes

Sat Apr 24 15:57:56 UTC 2010
Ross Walker <rswwalker at gmail.com>

On Apr 22, 2010, at 8:08 PM, Ray Van Dolson <rayvd at bludgeon.org> wrote:

> On Thu, Apr 22, 2010 at 03:57:01PM -0700, nate wrote:
>> John R Pierce wrote:
>>> Ray Van Dolson wrote:
>>>>> I think what you want is a proper storage array with mirrored  
>>>>> write
>>>>> cache.
>>>> Which is what we have with ZFS + SSD-based ZIL for far less money  
>>>> than
>>>> a NetApp.
>>> not unless you have a pair of them configured as an active/standby  
>>> HA
>>> cluster, sharing dual port disk storage, and some how (magic?)  
>>> mirroring
>>> the cache pool so that if the active storage controller/server  
>>> fails,
>>> the standby can take over wthout losing a single write.
>> OT too but really thought this was a good post/thread on ZFS
>> http://www.mail-archive.com/zfs-discuss@opensolaris.org/msg18898.html
>> "ZFS is designed for high *reliability*"
>> [..]
>> "You want something  completely different. You expect it to deliver
>> *availability*.
>> And availability is something ZFS doesn't promise. It simply can't
>> deliver this."
> Yep... and something you of course know going in.
> Don't want to get off on a tangent on that -- am still interested what
> type of solutions in the Linux world are out there that can  
> approximate
> what an SSD based ZIL does for ZFS.
> Kent Overstreet (from lkml) mentioned that his bcache patch is  
> intented
> to do something very similar.
> So I guess that's my answer -- it's not here yet, so sounds like the
> controller is the only way to achieve this currently.

How about locating XFS journal on SSDs and using HW RAID controller  
with big NVRAM cache.

That should be a lot faster than ZFS with SSD ZIL.

NFS should always be 'sync' if performance isn't good, then your  
storage isn't good.