Re: Very slow balance / btrfs-transaction

[Kai Krakow <hurikhan77@gmail.com>]

Am Mon, 6 Feb 2017 08:19:37 -0500
schrieb "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:

> > MDRAID uses stripe selection based on latency and other measurements
> > (like head position). It would be nice if btrfs implemented similar
> > functionality. This would also be helpful for selecting a disk if
> > there're more disks than stripesets (for example, I have 3 disks in
> > my btrfs array). This could write new blocks to the most idle disk
> > always. I think this wasn't covered by the above mentioned patch.
> > Currently, selection is based only on the disk with most free
> > space.  
> You're confusing read selection and write selection.  MDADM and
> DM-RAID both use a load-balancing read selection algorithm that takes
> latency and other factors into account.  However, they use a
> round-robin write selection algorithm that only cares about the
> position of the block in the virtual device modulo the number of
> physical devices.

Thanks for clearing that point.

> As an example, say you have a 3 disk RAID10 array set up using MDADM 
> (this is functionally the same as a 3-disk raid1 mode BTRFS
> filesystem). Every third block starting from block 0 will be on disks
> 1 and 2, every third block starting from block 1 will be on disks 3
> and 1, and every third block starting from block 2 will be on disks 2
> and 3.  No latency measurements are taken, literally nothing is
> factored in except the block's position in the virtual device.

I didn't know MDADM can use RAID10 on odd amounts of disks...
Nice. I'll keep that in mind. :-)


-- 
Regards,
Kai

Replies to list-only preferred.