Re: recommended way to add ssd cache to mdraid array

[Thomas Fjellstrom <thomas@fjellstrom.ca>]

On Fri Jan 11, 2013, you wrote:
> On Jan 11, 2013, at 5:20 AM, Thomas Fjellstrom <thomas@fjellstrom.ca> wrote:
> > On Thu Jan 10, 2013, Chris Murphy wrote:
> >> On Jan 10, 2013, at 3:49 AM, Thomas Fjellstrom <thomas@fjellstrom.ca> 
wrote:
> >>> A lot of it will be streaming. Some may end up being random
> >>> read/writes. The test is just to gauge over all performance of the
> >>> setup. 600MBs read is far more than I need, but having writes at 1/3
> >>> that seems odd to me.
> >> 
> >> Tell us how many disks there are, and what the chunk size is. It could
> >> be too small if you have too few disks which results in a small full
> >> stripe size for a video context. If you're using the default, it could
> >> be too big and you're getting a lot of RWM. Stan, and others, can
> >> better answer this.
> > 
> > As stated earlier, its a 7x2TB array.
> 
> OK fair enough, and as asked earlier, what's the chunk size?

Ah, sorry, I missed that bit. I didn't tweak the chunk size, so I have the 
default 512K.

> >> Another thing to look at is if you're using XFS, what your mount options
> >> are. Invariably with an array of this size you need to be mounting with
> >> the inode64 option.
> > 
> > I'm not sure, but I think that's the default.
> 
> It's not but as Stan writes it may not be preferred for your application.

Hm, ok, it is in my mount line, so I'll try it with that off. Though I would 
be interested in hearing from Stan why it may not be good.

> > Plain old seagate baracudas, so not the best but at least they aren't
> > greens.
> 
> They probably have a high ERC time out as all consumer disks do so you
> should also check /sys/block/sdX/device/timeout and make sure it's not
> significantly less than the drive. It may be possible for smartctl or
> hdparm to figure out what the drive ERC timeout is.
> 
> http://cgi.csc.liv.ac.uk/~greg/projects/erc/
> 
> > Debian seems to set up a weekly or monthly scrub, which I leave on due to
> > reading that same fact.
> 
> Unless there's a script to email you the results, you'll need to check it
> periodically. Or write a script that checks for reported errors and only
> sends an email if that's the case.

I believe it will send a mail to root. It seems as if its a mdadm/mdmon 
feature, as the config option is in mdadm.conf. I've set it to send a mail to 
my main email address, so I should see alerts.

> Chris Murphy
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html


-- 
Thomas Fjellstrom
thomas@fjellstrom.ca