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(a)athompso.net