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From: Ivan Voras <ivoras@freebsd.org>
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Subject: Re: ZFS performance degradation over time
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Andrew Snow wrote:
> Ivan Voras wrote:
>> It is true that ZFS in theory doesn't do very well with random writes 
>> of any kind - the kind that torrent clients do should actually be the 
>> worst case for ZFS, *but*, this very much depends on the actual workload.
> 
> 
> ZFS has aggressive read-ahead for sequential read-aheads, so its worth 
> noting that the performance problem can be mitigated by having lots of 
> RAM free for read-ahead, as well as multiple vdevs in the zpool (so that 
> it can be seeking all disks at once)

Yes and no. Read ahead will not help performance when the data is so 
fragmented that the disk is seek-bound. No matter how much of the file 
you can get in RAM, it still needs to be fetched from the drive 
platters. (Except if it's smart enough to read sequential chunks from 
the raw storage even though they are logically not located nearly, and 
in case of torrents, probably belong to different files, which I very 
much doubt it does).