ZFS on 10-STABLE r281159: programs, accessing ZFS pauses for minutes in state [*kmem arena]

[Paul Kraus]

On Jul 30, 2015, at 7:49, Steven Hartland <killing at multiplay.co.uk> wrote:

> On 30/07/2015 12:30, Lev Serebryakov wrote:

>> 
>>  Deduplication IS TURNED OFF. atime is turned off. Record size set to 1M as
>> I have a lot of big files (movies, RAW photo from DSLR, etc). Compression is
>> turned off.
> You don't need to do that as record set size is a min not a max, if you don't force it large files will still be stored efficiently.

Can you point to documentation for that ?

I really hope that the 128KB default is not a minimum record size or a 1KB file will take up 128 KB of FS space.

As far as I know, zfs recordsize has always, since the very beginning of ZFS under Solaris, been the MAX recrodsize, but it is also a hint and not a fixed value. ZFS will write any size records (powers of 2) from 512 bytes (4 KB in the case of an shift = 4 pool) up to recordsize. Tuning of recordsize has been frowned upon since the beginning unless you _know_ the size of your writes and they are fixed (like 8 KB database records). 

Also note that ZFS will fit the write to the pool in the case of RAIDz<n>, see Matt Ahrens bloig entry here: http://blog.delphix.com/matt/2014/06/06/zfs-stripe-width/

--
Paul Kraus
paul at kraus-haus.org