[RndTbl] Hybrid Flash+HDD - FYI

[Dan Martin]

I hadn't heard about hybrid drives.  Sounds like the cache that many mechanical drives have, but much larger than the typical 16 or 32 MB.

Is there a way of making your own hybrid?  It would be nice to have a 32GB SSD (or several in a RAID) that would act as a cache to slower drives.  Some logic would be needed to ensure that the most stale info is written to the mechanical HD when resources were available.

If the SSD array was sufficiently large, a cron job to periodically back it up to mechanical HD would be adequate.

The best solution would be a hardware device that would transparently use a SSD array as cache to a mechanical array.  Does such a thing exist?

-Dan

On 2011-05-21, at 12:30 PM, Adam Thompson wrote:

> Just an FYI.
> 
> I installed a hybrid HDD into my notebook (that's an ordinary HDD with 
> several GB's of flash designed to act as a cache).  The middle-tier cache 
> concept seems to work reasonably well, it keeps the most-seeked-to sectors 
> in flash for quick(er) access.  I can see a clear difference in disk 
> performance whenever my workload changes dramatically.
> Mostly, it just helps Windows boot faster, and helps MS Office apps load 
> faster.
> 
> But the Achilles' heel of any caching design like this is apparent when 
> you do something like, say, boot into another OS.  After running Ubuntu 
> for two days, rebooting into Windows felt sloooooow.  I guess two days was 
> enough time for full cache replacement, I think this model has 8GByte of 
> flash built in, so that's entirely reasonable.  Upon rebooting into 
> Windows, though, launching Outlook and opening a mailbox folder took 
> several *minutes* longer than normal.
> 
> It stands to reason that these types of disks would excel in areas where 
> there's strong locality of information... like the NTFS File Descriptor 
> table.  And this also explains why performance was utterly awful when 
> using btrfs - locality of metadata is almost zero with that FS.
> 
> So, just a hint if you're considering buying this type of disk - consider 
> your workload.  If your workload changes often, you'd be better off 
> spending the additional cash on a slightly faster or slightly bigger 
> non-hybrid HDD.
> 
> Is anyone using flash as a mid-tier cache in large disk arrays yet?  Do 
> they experience the same issue?
> 
> -Adam Thompson
> athompso at athompso.net
> 
> 
> 
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Dan Martin
GP Hospital Practitioner
Computer Scientist
ummar143 at shaw.ca
(204) 831-1746
answering machine always on