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Re: Q: RAID1 and chunk size

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I think it has no mean in RAID1 mode. It is used in RAID0,4,5,6,10 modes.

You can see in man mdadm.



Toni Mas

2018-03-07 23:06 GMT+01:00 Darac Marjal <mailinglist@darac.org.uk>:

>
>
> On 07/03/18 21:13, Steve Keller wrote:
> > I have a RAID1 array with 2 disks (/dev/sda1 and /dev/sdb1) of 2 TB
> > each.  By running mdadm -X /dev/sda1 I see that the chunk size is 64 MB:
> >
> >     # mdadm -X /dev/sda1
> >     Filename : /dev/sda1
> >     Magic : 6d746962
> >     Version : 4
> >     UUID : 300551ed:f6690dfb:1c939898:af5509c6
> >     Events : 2599997
> >     Events Cleared : 2599997
> >     State : OK
> >     Chunksize : 64 MB
> >     Daemon : 5s flush period
> >     Write Mode : Normal
> >     Sync Size : 1953381376 (1862.89 GiB 2000.26 GB)
> >     Bitmap : 29807 bits (chunks), 2 dirty (0.0%)
> >
> > What exactly does the chunk sized mean?  My question is how reads and
> > writes on an array are done.  Will the kernel always read or write a
> > complete chunk?  If so, does that mean that writing a single 4 KB
> > block to a file system will cause a 64 MB read, i.e one chunk, change
> > the 4 KB block in that chunk and write back the 64 MB chunk?
>
> Yes, my understanding is that chunk size is the size of area upon which
> parity is calculated, or the size of data which is allocated before
> moving onto the next drive etc.
>
> My guess, though, is that there is a balance to be struck. Yes, if the
> chunk size is small, then there is very little write amplification. But
> if the chunk size is too small, then you need to wait for that chunk to
> pass the read-write head again, you need to be switching between sectors
> very often etc. With a bigger chunk, you can take better advantage of
> caching. These days, 64Mb is a relatively small amount to pull into a
> buffer, it can be pulled in, modified and rewritten virtually
> instantanously.
>
> There's a nice article on the effect of different chunk sizes here:
> http://louwrentius.com/linux-raid-level-and-chunk-size-the-benchmarks.html
>
> >
> > Wouldn't that mean a massive performance problem?
> >
> > Steve
> >
>
>
>

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Q: RAID1 and chunk size "Steve Keller" <keller.steve@gmx.de> - 2018-03-07 22:40 +0100
  Re: Q: RAID1 and chunk size Darac Marjal <mailinglist@darac.org.uk> - 2018-03-07 23:10 +0100
    Re: Q: RAID1 and chunk size Toni Mas i Soler <antomassol@gmail.com> - 2018-03-08 11:00 +0100

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