Josef Bacik <jba...@fb.com> schrieb:
>
> On 02/05/2014 03:15 PM, Roman Mamedov wrote:
>> Hello,
>>
>> On a freshly-created RAID1 filesystem of two 1TB disks:
>>
>> # df -h /mnt/p2/
>> Filesystem Size Used Avail Use% Mounted on
>> /dev/sda2 1.8T 1.1M 1.8T 1% /mnt/p2
>>
>> I cannot write 2TB of user data to that RAID1, so this estimate is
>> clearly misleading. I got tired of looking at the bogus disk free space
>> on all my RAID1 btrfs systems, so today I decided to do something about
>> this:
>>
>> --- fs/btrfs/super.c.orig 2014-02-06 01:28:36.636164982 +0600
>> +++ fs/btrfs/super.c 2014-02-06 01:28:58.304164370 +0600
>> @@ -1481,6 +1481,11 @@
>> }
>>
>> kfree(devices_info);
>> +
>> + if (type & BTRFS_BLOCK_GROUP_RAID1) {
>> + do_div(avail_space, min_stripes);
>> + }
>> +
>> *free_bytes = avail_space;
>> return 0;
>> }
>
> This needs to be more flexible, and also this causes the problem where
> now you show the actual usable amount of space _but_ you are also
> showing twice the amount of used space. I'm ok with going in this
> direction, but we need to convert everybody over so it works for raid10
> as well and the used values need to be adjusted. Thanks,
It should show the raw space available. Btrfs also supports compression and
doesn't try to be smart about how much compressed data would fit in the free
space of the drive. If one is using RAID1, it's supposed to fill up with a
rate of 2:1. If one is using compression, it's supposed to fill up with a
rate of maybe 1:5 for mostly text files.
However, btrfs should probably provide its own df utility (like "btrfs fi
df") which is smart about disk usage and tries to predict the usable space.
But df should stay with actually showing the _free_ space, not _usable_
space (the latter is unpredictable anyway based on the usage patterns
applied to the drive, so it can be a rough guess as its best, like looking
into the crystal ball).
The point here is about "free" vs. "usable" space, the first being a known
number, the latter only being a prediction based on current usage. I'd like
to stay with "free space" actually showing raw free space.
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