[vdr] mdadm software raid5 arrays?
udo_richter at gmx.de
Sun Nov 22 01:44:32 CET 2009
On 18.11.2009 18:28, H. Langos wrote:
> I/O-load can have some nasty effects. E.g. if your heads have to jump
> back and forth between an area from where you are reading and an area
> to which you are recording.
I remember reading some tests about file system write strategies that
showed major differences between file systems when writing several file
streams in parallel. IIRC the old EXT2/3 was way at the lower end, while
XFS scored more at the upper end.
One major point here is to avoid heavy seeking, by massive use of write
caching and read ahead caching. Another one is a smart allocation
strategy so that the files don't get interleaved too much, and so that
metadata doesn't have to be read/written too often. (-> extents)
> In a raid1 setup you have two sets of heads that you can work with.
> (Or more if you are willing to put in more disks.)
In theory yes, but I would really like to know whether raid systems are
actually smart enough to split their read operations between the heads
in an efficient way. For example, while reading two data streams, its
probably the best to use one head for each stream. Unless one of the
streams needs a higher bandwidth, in which case it would be more wise to
use one head exclusively, and let the other jump between the streams.
And what if there are several small reads in parallel? Which head should
In the end you can probably put a lot of strategy fine tuning into this,
and there will still be situations where a different strategy would
still improve performance in some scenarios - or in others not.
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