Re: Reports from SSD purgatory

On Wed, 24 Aug 2011, Merlin Moncure wrote:

> On Wed, Aug 24, 2011 at 1:48 PM, <gnuoytr(at)rcn(dot)com> wrote:
>>
>>
>>
>> Also, given that PG is *nix centric and support for TRIM is win
>> centric, having that makes a big difference in performance.
>
> one point about TRIM -- no raid controller that I know of supports
> trim, which suggests it might not even be possible to support. How
> much does it help really? Probably not as much as you would think
> because newer SSD drives have very sophisticated controllers that make
> it at least partially obsolete.

if the SSD can know that the user doesn't care about data in a particular
block, the SSD can overwrite that block with new data.

Since the SSDs do their writing to new blocks and erase old blocks later,
the more empty blocks you have available, the less likely you are to hit a
garbage collection pause when you try to write to the drive.

if you are careful to never write temporary files to the drive and only
use it for database-like 'update in place' type of things (no 'write a new
file and then rename it over the old one' tricks), then TRIM won't make
any difference because every block that you have ever written to is one
that you care about (or close enough to this for practical purposes)

but if you don't take this care, the drive works to preserve all the data
blocks that you have ever written to, even if the filesystem has freed
them and dosn't care about them. The worst case would be a log strcutured
filesystem (btrfs for example) where every write is to a new block and
then the old block is freed later.

David Lang