ZFS on top of GELI

[jhell]

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On Mon, 11 Jan 2010 19:45, fjwcash@ wrote:
> On Mon, Jan 11, 2010 at 4:24 PM, Dan Naumov <dan.naumov at gmail.com> wrote:
>
>> On Tue, Jan 12, 2010 at 1:29 AM, K. Macy <kmacy at freebsd.org> wrote:
>>>>>
>>>>> If performance is an issue, you may want to consider carving off a
>> partition
>>>>> on that SSD, geli-fying it, and using it as a ZIL device.  You'll
>> probably
>>>>> see a marked performance improvement with such a setup.
>>>>
>>>> That is true, but using a single device for a dedicated ZIL is a huge
>>>> no-no, considering it's an intent log, it's used to reconstruct the
>>>> pool in case of a power failure for example, should such an event
>>>> occur at the same time as a ZIL provider dies, you lose the entire
>>>> pool because there is no way to recover it, so if ZIL gets put
>>>> "elsewhere", that elsewhere really should be a mirror and sadly I
>>>> don't see myself affording to use 2 SSDs for my setup :)
>>>>
>>>
>>> This is  false. The ZIL is used for journalling synchronous writes. If
>>> your ZIL is lost you will lose the data that was written to the ZIL,
>>> but not yet written to the file system proper. Barring disk
>>> corruption, the file system is always consistent.
>>>
>>> -Kip
>>
>> Ok, lets assume we have a dedicated ZIL on a single non-redundant
>> disk. This disk dies. How do you remove the dedicated ZIL from the
>> pool or replace it with a new one? Solaris ZFS documentation indicates
>> that this is possible for dedicated L2ARC - you can remove a dedicated
>> l2arc from a pool at any time you wish and should some IO fail on the
>> l2arc, the system will gracefully continue to run, reverting said IO
>> to be processed by the actual default built-in ZIL on the disks of the
>> pool. However the capability to remove dedicated ZIL or gracefully
>> handle the death of a non-redundant dedicated ZIL vdev does not
>> currently exist in Solaris/OpenSolaris at all.
>>
>> That has been implemented in OpenSolaris, do a search for "slog removal".
> It's in a much newer zpool version than 13, though.
>
>

What I have seen more often by users is taking the usage of slog/ZIL 
wrong. For instance dedicating a whole SSD or another HDD as the slog. 
Your slog/ZIL only has to be big enough to handle 10 seconds of synchronous 
writes before it flushes. A recommended ZIL from Sun Micro is 128MB but 
you may not even see that fully used for general purpose cases.

I had is to dedicate a partition on the same disk that the pool is on and 
adding another ZIL vdev from another disk in the system. Results of this 
imply that if the off-disk ZIL dies for some stupid reason it falls back 
to the one that rests on the same disk as the pool and allows to replace 
the off-disk ZIL with something else.

PS: Save your disk space and use 256MB thumb drives. you can easily get 16 
of those at your local Walmart and have a priceless light show for a 
romantic dinner with the wife.

  :)

- -- 

  Mon Jan 11 22:31:17 2010

  jhell

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