Hi, Marc Raid0 is not redundant in any way. See inline below.
On 2014/05/04 01:27 AM, Marc MERLIN wrote:
So, I was thinking. In the past, I've done this: mkfs.btrfs -d raid0 -m raid1 -L btrfs_raid0 /dev/mapper/raid0d* My rationale at the time was that if I lose a drive, I'll still have full metadata for the entire filesystem and only missing files. If I have raid1 with 2 drives, I should end up with 4 copies of each file's metadata, right? But now I have 2 questions 1) btrfs has two copies of all metadata on even a single drive, correct?
Only when *specifically* using -m dup (which is the default on a single non-SSD device), will there be two copies of the metadata stored on a single device. This is not recommended when using multiple devices as it means one device failure will likely cause critical loss of metadata. When using -m raid1 (as is the case in your first example above and as is the default with multiple devices), two copies of the metadata are distributed across two devices (each of those devices with a copy has only a single copy).
If so, and I have a -d raid0 -m raid0 filesystem, are both copies of the metadata on the same drive or is btrfs smart enough to spread out metadata copies so that they're not on the same drive?
This will mean there is only a single copy, albeit striped across the drives.
2) does btrfs lay out files on raid0 so that files aren't striped across more than one drive, so that if I lose a drive, I only lose whole files, but not little chunks of all my files, making my entire FS toast?
"raid0" currently allocates a single chunk on each device and then makes use of "RAID0-like" stripes across these chunks until a new chunk needs to be allocated. This is good for performance but not good for redundancy. A total failure of a single device will mean any large files will be lost and only files smaller than the default per-disk stripe width (I believe this used to be 4K and is now 16K - I could be wrong) stored only on the remaining disk will be available.
The scenario you mentioned at the beginning, "if I lose a drive, I'll still have full metadata for the entire filesystem and only missing files" is more applicable to using "-m raid1 -d single". Single is not geared towards performance and, though it doesn't guarantee a file is only on a single disk, the allocation does mean that the majority of all files smaller than a chunk will be stored on only one disk or the other - not both.
Thanks, Marc
I hope the above is helpful. -- __________ Brendan Hide http://swiftspirit.co.za/ http://www.webafrica.co.za/?AFF1E97 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
