Re: RAID 0 across SSD and HDD

[Hugo Mills <hugo@carfax.org.uk>]

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On Wed, Jan 30, 2013 at 01:27:37AM -0800, Roger Binns wrote:
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> I've been unable to find anything definitive about what happens if I use
> RAID0 to join an SSD and HDD together with respect to performance
> (latency, throughput).  The future is obvious (hot data tracking, using
> most appropriate device for the data, data migration).
> 
> In my specific case I have a 250GB SSD and a 500GB HDD, and about 250GB of
> files (constantly growing).  One message I saw said that new blocks are
> allocated on the device with the most free space which implies the SSD
> would be virtually unused in my case, except for metadata which would only
> be used half the time.

   That would be the case with "single" mode, not with RAID-0.

   With RAID-0, you'd get data striped equally across all (in this
case, both) the devices, up to the size of the second-largest one, at
which point it'll stop allocating space.

> At the moment I have two independent filesystems (one per device) and
> manually move data files between them using symlinks to keep pathnames the
> same.  This requires keeping lots of slop free space on the SSD as well as
> administration whenever it runs out of space.
> 
> My hope would be overall performance between that of the two devices, and
> closer to that of the SSD.

   We don't have any kind of hot-data management yet, but it's on the
list of things we'd like to have at some point.

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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