Re: [BUG] can not allocate space for caching data

[Miao Xie <miaox@cn.fujitsu.com>]

On Mon, 20 Dec 2010 07:44:06 -0500, Chris Mason wrote:
> Excerpts from Miao Xie's message of 2010-12-20 07:25:10 -0500:
>> Hi, Chris
>>
>> There is something wrong with this patch:
>>
>> commit 83a50de97fe96aca82389e061862ed760ece2283
>> Author: Chris Mason<chris.mason@oracle.com>
>> Date:   Mon Dec 13 15:06:46 2010 -0500
>>
>>      Btrfs: prevent RAID level downgrades when space is low
>>
>>      The extent allocator has code that allows us to fill
>>      allocations from any available block group, even if it doesn't
>>      match the raid level we've requested.
>>
>>      This was put in because adding a new drive to a filesystem
>>      made with the default mkfs options actually upgrades the metadata from
>>      single spindle dup to full RAID1.
>>
>>      But, the code also allows us to allocate from a raid0 chunk when we
>>      really want a raid1 or raid10 chunk.  This can cause big trouble because
>>      mkfs creates a small (4MB) raid0 chunk for data and metadata which then
>>      goes unused for raid1/raid10 installs.
>>
>>      The allocator will happily wander in and allocate from that chunk when
>>      things get tight, which is not correct.
>>
>>      The fix here is to make sure that we provide duplication when the
>>      caller has asked for it.  It does all the dups to be any raid level,
>>      which preserves the dup->raid1 upgrade abilities.
>>
>>      Signed-off-by: Chris Mason<chris.mason@oracle.com>
>>
>> Btrfs has added the space of single chunks and raid0 chunks into the space
>> information, so when we use btrfs_check_data_free_space() to check if there
>> is some space for storing file data, this function may return true. So we
>> write the data into the cache successfully. But, the extent allocator can
>> not allocate any space to store that cached data, and then the file system
>> panic.
>>
>> I think we subtract that space from the space information, or split the space
>> information into two types, one is used to manage the chunks with duplication,
>> the other manages the other chunks.
>
> Ok, do you have a test case that triggers this?  I'll work out a patch.
> Yan Zheng's original idea of 'the chunks should be readonly' should help
> us deduct them from the total.

# mkfs.btrfs -d raid1 /dev/sda9 /dev/sda10
# mount /dev/sda9 /mnt
# dd if=/dev/zero of=/mnt/tmpfile0 bs=4K count=999999999999999999
   (fill the file system)
# umount /mnt
# mount /dev/sda9 /mnt
# dd if=/dev/zero of=/mnt/tmpfile1 bs=4K count=1000
# sync

Thanks
Miao