Re: [gentoo-user] Re: ZFS

[Pandu Poluan <pandu@poluan.info>]

On Thu, Sep 19, 2013 at 8:01 PM, Grant <emailgrant@gmail.com> wrote:
>>>> You should definitely determine the right value for ashift on pool
>>>> creation
>>>> (it controls the alignment on the medium). It's an option that you afaik
>>>> can only set
>>>> on filesystem creation and therefore needs a restart from scratch if you
>>>> get it
>>>> wrong.
>>>> According to the illumos wiki it's possible to run a mixed pool (if you
>>>> have
>>>> drives requiring different alignments[1])
>>>> If in doubt: ask ryao (iirc given the right information he can tell you
>>>> which
>>>> are the right options for you if you can't deduce it yourself).
>>>> Choosing the wrong alignment can cause severe performance loss (that's not
>>>> a ZFS issue but happened when 4k sector drives appeared and tools like
>>>> fdisk
>>>> weren't aware of this).
>>>
>>> Yikes...
>>>
>>> Ok, shouldn't there be a tool or tools to help with this? Ie, boot up on a
>>> bootable tools disk on the system with all drives connected, then let it
>>> 'analyze' your system, maybe ask you some questions (ie, how you will be
>>> configuring the drives/RAID, etc), then spit out an optimized config for
>>> you?
>>>
>>> It is starting to sound like you need to be a dang engineer just to use
>>> ZFS...
>>>
>>
>> Just do ashift=12 and you're good to go. No need to analyze further.
>>
>> The reason I said that because in the future, *all* drives will have 4
>> KiB sectors. Currently, many drives still have 512 B sectors. But when
>> one day your drive dies and you need to replace it, will you be able
>> to find a drive with 512 B sectors?
>>
>> Unlikely.
>>
>> That's why, even if your drives are currently of the 'classic' 512 B
>> ones, go with ashift=12 anyway.
>>
>> For SSDs, the situation is murkier. Many SSDs 'lie' about their actual
>> sector size, reporting to the OS that their sector size is 512 B (or 4
>> KiB). No tool can pierce this veil of smokescreen. The only way is to
>> do research on the Internet.
>
> OK, so figure out what SSD you're using and Google to find the correct ashift?
>
> - Grant
>

Kind of like that, yes. Find out exactly the size of the SSD's
"internal sectors" (for lack of better term), and find the log2 to it.

But don't go higher than ashift=14

Rgds,
-- 
FdS Pandu E Poluan
~ IT Optimizer ~

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