Secure Erase is currently a entire drive function, its writes all the cell
resetting it. It also updates the firmware GC maps so it knows the drive is
clean. Trim just gives more info the firmware that a block is unused (as
normally a delete is just updating an index table and the firmware has no
way of knowing which cells are no longer needed by the OS).
Currently firmware is meant to help conventional file system usage. However
ZIL isn't normal usage and as such *IF* and it's a big if, we can
effectively bypass the firmware trying to be clever or at least help it be
clever then we can avoid the downgrade over time. In particular if we could
secure erase a few cells as once as required, the lifetime would be much
longer, I'd even argue that taking the wear leveling off the drives hand
would be useful in the ZIL case. The other thing is the slow down only
occurs once the SSD fills and has to start getting clever where to put
things and which cells to change, for a ZIL that is again something we could
avoid in software fairly easy.
Its also worth putting this in perspective, a complete secure erase every
night to restore performance to your ZIL would still let the SSD last for
*years*. And given how cheap some SSD are, it is probably still cheaper to
effectively burn the ZIL out and just replace it once a year. Maybe not a
classic level of RAID but the very essence of the idea, lots of cheap can be
better than expensive if you know what you are doing.
[mailto:firstname.lastname@example.org] On Behalf Of Christopher George
Sent: 23 December 2010 16:46
Subject: Re: [zfs-discuss] Looking for 3.5" SSD for ZIL
> However, this *can* be overcome by frequently re-formatting the SSD (not
> the Solaris format, a low-level format using a vendor-supplied utility).
For those looking to "Secure Erase" a OCZ SandForce based SSD to reclaim
performance, the following OCZ Forum thread might be of interest:
OCZ uses the term "DuraClass" as a catch-all for algorithms controlling wear
leveling, drive longevity... There is a direct correlation between Secure
frequency and expected SSD lifetime.
Thread #1 detailing a recommended frequency of Secure Erase use:
"3) Secure erase a drive every 6 months to free up previously read only
blocks, secure erase every 2 days to get round Duraclass and you will kill
drive very quickly"
Thread #5 explaining DuraClass and relationship to TRIM:
"Duraclass is limiting the speed of the drive NOT TRIM. TRIM is used along
with wear levelling."
Thread #6 provides more details of DuraClass and TRIM:
"Now Duraclass monitors all writes and control's encryption and compression,
this is what effects the speed of the blocks being written to..NOT the fact
have been TRIM'd or not TRIM'd."
"You guys have become fixated at TRIM not speeding up the drive and forget
that Duraclass controls all writes incurred by the drive once a GC map has
Above excerpts written by a OCZ employed thread moderator (Tony).
This message posted from opensolaris.org