On Wed, Nov 13, 2013 at 6:13 AM, Matthew Patton <[email protected]> wrote:
> On Wed, 13 Nov 2013 03:42:54 -0500, Francis Moreau <[email protected]>
> wrote:
>
>> First I'd like to know if over-provisioning is still needed when
>> bcache is used ?
>
>
> it's needed for any decent, sustained write load if you don't want
> performance to crash thru the floor. But it doesn't sound like this is your
> use case. Are you sustaining 5000+ write IOPs?
>
>
>> - Should I create 2 partitions with one left emtpy and whose size is
>> roughly 7% of the device size ?
>
bcache has a pretty flash friendly write pattern, so you probably
don't need a huge lot of empty space. The 7% might in fact be enough,
but bumping it up to 10%-12% wouldn't hurt. I think the >25% numbers
are more relevant for non-flash friendly writes (like random writes).
In that case you definitely want lots of free space.
>
> Your SSD already has 7% overprovisioning. For best sustained and consistent
> write performance you'll want to bump it to 25-30%. Yes you just need to
> partition/LVM down to the desired size.
>
>
>> - Should I create a "Host Protected Area" by using hdparm ?
>
>
> http://www.thomas-krenn.com/en/wiki/SSD_Over-provisioning_using_hdparm
>
> Either take the precautions outlined in the WIKI given the penchant of the
> kernel to ignore such settings, or just remove the chance for error and
> partition the device instead.
>
>
>> And finally, can I create the over-provisioning without lost the
>> content of my backing device ?
>
>
> hell no.
>
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