Re: RAID5 created by 8 disks works with xfs

[David Brown <david@westcontrol.com>]

On 01/04/2012 14:28, John Robinson wrote:
> On 01/04/2012 11:33, David Brown wrote:
> [...]
>> For an application like this, it would probably make sense to put the
>> xfs log (and the mdraid bitmap file, if you are using one) on a separate
>> disk - perhaps a small SSD.
>
> Now that you've suggested that, it occurs to me that for an application
> like this, the OP might be better doing his multiple slow streams to a
> spool folder on SSD, and copying over to the big array of spinning rust
> when each stream completes. 200 streams of 500MB is 100GB of data, so a
> pair of slightly larger SSDs in RAID1 (or RAID10, to balance the reads
> coming off) would do nicely as a spool area.

If the OP wants to do that, he must be careful when looking at the rated 
write speeds for the SSDs.  SSDs need a bit of a break on occasion to 
get time to do their garbage collection - their long-term streamed write 
speed is therefore lower than their short-term write speeds (and that's 
the figure that will be given in the spec).  But if you take that into 
account, and short-partition the SSD's a little (i.e., leave 10% 
unpartitioned, and put the raid1 on the partition rather than the disk) 
so that there are always plenty of free blocks when they are needed, 
that could be a very good solution.

>
> This might also be a good application for bcache, FlashCache or whatever.
>
> Cheers,
>
> John.
>