Re: brtfs on top of dmcrypt with SSD -> Trim or no Trim

[Marc MERLIN <marc@merlins.org>]

On Thu, Jul 19, 2012 at 03:04:29AM +0700, Fajar A. Nugraha wrote:
> > On the Crucial RealSSD C300 256GB, and from talking to their tech support
> > and other folks who happened to have gotten that 'drive' at work and also
> > got weird unexplained failures, I'm convinced that even its latest 007
> > firmware (the firmware it shipped with would just hang the system for a few
> > seconds every so often so I did upgrade to 007 early on), the drive does
> > very poorly without TRIM when it's getting close to full.
> 
> If you're going to edit the wiki, I'd suggest you say "SOME SSDs might
> need to use TRIM with dmcrypt". That's because some SSD controllers
> (e.g. sandforce) performs just fine without TRIM, and in my case TRIM
> made performance worse.

Right. I'm definitely planning on writing that it is controller dependent.

By the way, I read that SF performs worse with dmcrypt because:
http://superuser.com/questions/250626/sandforce-ssd-encryption-security-and-support
'Using software full-disk-encryption on an SSD seriously impacts the
performance of the drive - up to a point where it can be slower than a
regular hard disk.'
http://c089.wordpress.com/2011/02/28/encrypting-solid-state-disks/
http://www.madshrimps.be/articles/article/965#axzz1FGfzD15q

So, really, it seems that one should not buy a drive with a sandforce
controller if you plan to use dmcrypt.
I haven't found clear data to say which controllers other than sandforce
have the same problem.

https://docs.google.com/a/google.com/viewer?a=v&q=cache:eyoe3gL10qcJ:www.samsung.com/global/business/semiconductor/file/product/ssd/SamsungSSD_Encryption_Benchmarks_201011.pdf+&hl=en&gl=us&pid=bl&srcid=ADGEEShHEj1jkxdwa7JREmubW6iAyc2RGqQt8MoGxMgQrRNDifoGqTYWjYdaUypnaRtkjKrjiOt1JCr4dDt4ycD2rjWO51PtAo67JvnZGe6Gx1s-9yjkRVYZWsUHZApkjm7dWVzkdQg6&sig=AHIEtbRaViqG4cYC_RYajAW04JRjNhQq6g
seems to imply that software encryption on samsung drives (which is what I
just got as a replacement for my crucial), does not have the same penalty
than you have with the SF controller.

But back to the point about "yes, it depends".

Marc
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