Date: Tue, 29 Jan 2013 10:58:40 -0000 From: "Steven Hartland" <killing@multiplay.co.uk> To: "Adam Nowacki" <nowakpl@platinum.linux.pl>, "Matthew Ahrens" <mahrens@delphix.com> Cc: fs@freebsd.org Subject: Re: RAID-Z wasted space - asize roundups to nparity +1 Message-ID: <2FD375DC62B24754B8945BF0A1E26B78@multiplay.co.uk> References: <5105252D.6060502@platinum.linux.pl> <CAJjvXiEQSqnKYP75crTkgVqLKSk92q9UTikFtdyPHmF6shJFbg@mail.gmail.com> <5107A9B7.5030803@platinum.linux.pl>
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----- Original Message -----
From: "Adam Nowacki" <nowakpl@platinum.linux.pl>
> On 2013-01-28 22:55, Matthew Ahrens wrote:
>> This is so that we won't end up with small, unallocatable segments.
>> E.g. if you are using RAIDZ2, the smallest usable segment would be 3
>> sectors (1 sector data + 2 sectors parity). If we left a 1 or 2 sector
>> free segment, it would be unusable and you'd be able to get into strange
>> accounting situations where you have free space but can't write because
>> you're "out of space".
>
> Sounds reasonable.
>
>> The amount of waste due to this can be minimized by using larger
>> blocksizes (e.g. the default recordsize of 128k and files larger than
>> 128k), and by using smaller sector sizes (e.g. 512b sector disks rather
>> than 4k sector disks). In your case these techniques would limit the
>> waste to 0.6%.
>
> This brings another issue - recordsize capped at 128KiB. We are using
> the pool for off-line storage of large files (from 50MB to 20GB). Files
> are stored and read sequentially as a whole. With 12 disks in RAID-Z2,
> 4KiB sectors, 128KiB record size and the padding above 9.4% of disk
> space goes completely unused - one whole disk.
This is something thats being worked on upstream, its not as trivial
as it first looks unfortuantely.
Regards
Steve
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