[RndTbl] Hybrid Flash+HDD - FYI

Adam Thompson athompso at athompso.net
Sat May 21 12:30:47 CDT 2011

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 

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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