On 04/20/2014 01:54 PM, Chris Murphy wrote:
This is expected. And although I haven't tested it, I think you'd get
the same results with multiple threads writing at the same time: the
allocation would aggregate the threads to one chunk at a time until
full, which means writing to one device at a time, then writing a new
chunk on a different device until full, and so on in round robin
fashion.
Interesting and some what shocked -- if I am reading this correctly!
So ... BTRFS at this point in time, does not actually "stripe" the data
across N number of devices/blocks for aggregated performance increase
(both read and write)?
Essentially running mdadm with ext4 or XFS would offer better
performance then BTRFS right now (and possible the ZFS on Linux project)?
I think I may be missing a keypoint here (or not RTFM)?
The WiKi page[1] clearly shows that the command I used to create my
current setup (-m single) will *not* stripe the data which should have
been the first _red_ flag for me! However, if I go ahead and create using
mkfs.btrfs -d raid0 /dev/sda3 /dev/sdb /dev/sdc
This *should* stripe the data and improve read and write performance?
But according to what Chris wrote above, this is not true? Just want
some clarification on this.
So my question is, should I have setup the BTRFS filesystem with -d
raid0? Would this have worked with multiple devices with different
sizes?
raid0 does work with multiple devices of different sizes, but it
won't use the full capacity of the last drive with the most space.
For example: 2GB, 3GB, and 4GB devices as raid0.
The first 2GB copies using 3 stripes, one per device, until the 2GB
device is full. The next 1GB copies using 2 stripes, one per
remaining device (the 3GB and 4GB ones) until the 3GB device is full.
Additional copying results in "cp: error writing ‘./IMG_2892.dng’: No
space left on device"
I am sorry, I do not quite understand this. If I read this correctly, we
are copying a file that is larger then the total raid0 filesystem
(9GB?). The point at which writes fail is at the magic number of 5GB --
which is where the two devices are full?
So going back to the setup I currently have:
Label: none uuid: 63d51c9b-f851-404f-b0f2-bf84d07df163
Total devices 3 FS bytes used 3.03TiB
devid 1 size 3.61TiB used 1.01TiB path /dev/sda3
devid 2 size 3.64TiB used 1.04TiB path /dev/sdb
devid 3 size 3.64TiB used 1.04TiB path /dev/sdc
If /dev/sda3 and /dev/sdb are full, but room is still left on /dev/sdc,
writes file -- but the metadata will continue to succeed taking up
ionodes and creating zero length files?
[1]:
https://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices
--
Adam Brenner <[email protected]>
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