3.5in SAS flash drive for R900?

Tim Small tim at seoss.co.uk
Fri Sep 10 04:53:44 CDT 2010


On 09/09/10 18:57, Philip Tait wrote:
>
>     Will the PERC-6i do TRIM passthrough?  If not, you'll need to use an
>     AHCI controller such as the server's onboard Intel SATA controller
>     etc.
>     (this is what we are using).
>
>     Does your kernel and filesystem support TRIM too?
>
>
>
> Is the availability of TRIM support critical to the operation of these 
> drives?

I believe all SSDs will suffer degraded performance over time, and/or 
excessive wear without TRIM - they end up doing extra writes in order to 
(unnecessarily) preserve the contents of deleted logical blocks.  Same 
goes for running them pretty-much full all the time with TRIM.  However 
the Intel drives suffer less from a lack of TRIM support than some other 
designs do (i.e. performance degrades, but doesn't drop through the floor).

This is basically due to the current generation of SSDs pretending a 
hard disk - when really they are flash (which has some very different 
physical properties).  Probably the best solution from an engineering 
point of view would be to use a flash-file-system (Linux has several - 
and they are designed to suite the physical properties of flash 
storage), and have the SSDs expose the raw underlying flash.  This 
however, is not the route the industry is taking....  TRIM is a piece of 
gaffer/duct-tape to fix this problem by providing a mechanism for the OS 
to tell the SSD which logical blocks it no-longer needs to work to 
preserve....

Without TRIM a workaround is to periodically discard all data by doing 
an ATA security-erase-unit, but this might not fit in with your 
anticipated usage.

bcache also looks very interesting, but is currently alpha-quality  
http://bcache.evilpiepirate.org/

BTW, the machine I'm using currently uses TRIM with both ext4, and btrfs 
on an Intel SSD, with an AHCI controller (Intel ICH10).

Tim.


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