Date: Mon, 11 Apr 2011 10:47:27 -0400 From: "Daniel Staal" <DStaal@usa.net> To: freebsd-questions@freebsd.org Subject: Re: ZFS Striping and Optimizing Capabilities Message-ID: <b52c7b2805fadef2b7d832229828cc3a.squirrel@www.magehandbook.com> In-Reply-To: <20110410063258.GA10022@dan.emsphone.com> References: <4DA131BD.3030805@telting.org> <20110410063258.GA10022@dan.emsphone.com>
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On Sun, April 10, 2011 2:32 am, Dan Nelson wrote:
> In the last episode (Apr 09), Chris Telting said:
>> Just a few questions about what ZFS actually does. So if anyone has
>> intimate knowledge about ZFS's implementation on Freebsd I'm sure I and
>> others would appreciate the answers.
>>
>> When you add a second and or thrid drive/partition to a zpool I'm
>> assuming that it's going to start using the drives like a raid 0
>> stripe. How do the ZFS versions differ in this? Does it immediately
>> start striping all files in the background on low priority or does it
>> do it as files are accessed? Does ZFS in any way do performance
>> testing of
>
> Currently ZFS never moves a block once it has been written. The whole
> implementation of copy-on-write and snapshots relies on this. If you add
> a new zvol to a pool, new data will get balanced across all the zvols
> but old data will stay where it was. Ideally you should expand pools
> before they get too full, or zpool export/import them into new pools if
> you want good distribution over all disks. There have been hints for the
> last few years of a "bp rewite" feature appearing, but it has never
> materialized.
One thought that hasn't been mentioned, and probably should be in this
context: It depends a bit on how you add the disk. A disk can be added
to a non-RAIDZ zpool as either a stripe or a mirror. If you add it as a
mirror ('attach' in zpool terms), then yes it starts a background low
priority process that copies data over.
I don't really think that was your question, but it's closely enough
related that I thought it might be relevant. In the context of stripes it
probably isn't relevant. But it could be in the context of adding disks.
Daniel T. Staal
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