Question about ZFS with log and cache on SSD with GPT

[Alexander Leidinger]

Quoting Willem Jan Withagen <wjw at digiware.nl> (from Sun, 22 Jan 2012  
17:13:11 +0100):

> On 22-1-2012 9:10, Peter Maloney wrote:
>> Am 21.01.2012 23:06, schrieb Alexander Leidinger:
>>>> Corsair reports:
>>>>> Max Random 4k Write (using IOMeter 08): 50k IOPS (4k aligned)
>>>>> So I guess that suggests 4k aligned is required.
>>> Sounds like it is.
>>>
>> I'm not an SSD expert, but I read as much as I can, and found that many
>> say that the sector size is not the only thing that matters on an SSD,
>> but also the *erase boundary*. The size of the erase boundary varies,
>> but 2MiB is a common factor (or 1MiB for 99% of them), so you can use
>> that for all.
>>
>> The theory I read about is that when the SSD wants to write something,
>> it must erase the whole erase block first. If it needs to erase a whole
>> erase boundary space to write 512 bytes, that is just normal. But if you
>> are misaligned, it often needs to erase 2 erase boundary spaces.
>>
>> Here is an example from our FreeBSD forum:
>> http://forums.freebsd.org/showthread.php?t=19093
>
> Thanx for this thread, there is a lot of usefull info there.
> pithy thing is to blow 66Mb, but then again on 40 or 120 Gb SSDs it is
> only marginal. (Guess it stems from the time that HDs where 5Mb :) )
>
> I'm still not really shure that that is needed it the bios has nothing
> to do with these disks, as in our case: SSDs are only used as caches
> under ZFS.

I think the erase boundary only matters for speed, if the FS in  
question really deletes blocks in disk, instead of just "not using it  
anymore". I was told a while ago that ZFS is not doing BIO_DELETE,  
specially not on cache devices. So I do not expect that you will see  
an improvement by taking the erease boundary into account (except your  
SSD has not a decent wear-leveling implementation).

Bye,
Alexander.

-- 
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