converting to raid5

converting to raid5

[Remco Hosman]

Hi,

Just installed 3.9.0-rc2 and the latest btrfs-progs. 

filesystem is a 4 disk raid1 array.

first, i did the following: `btrfs val start -dconvert=raid5,usage=1` to convert the mostly empty chunks.
This resulted in a lot of allocated space (10's of gigs), with only a few 100 meg used.
i did `btrfs val start -dusage=75` to clean things up.

then i ran `btrfs bal start -dconvert=raid5,soft`.
I noticed how the difference between total and used for raid5 kept growing. 
My guess is that its taking 1 raid1 chunk (2x1 gig disk space, 1 gig data), and moving it to 1 raid5 chunk (4gig disk space, 3gig data), leaving all chunks 33% used.

This is what 3 calls of `btrfs file df /` looks like a few minutes after each other, with the balance still running:

Data, RAID1: total=807.00GB, used=805.70GB
Data, RAID5: total=543.00GB, used=192.81GB
System, RAID1: total=32.00MB, used=192.00KB
Metadata, RAID1: total=6.00GB, used=3.54GB
--
Data, RAID1: total=800.00GB, used=798.70GB
Data, RAID5: total=564.00GB, used=199.30GB
System, RAID1: total=32.00MB, used=192.00KB
Metadata, RAID1: total=6.00GB, used=3.53GB
--
Data, RAID1: total=795.00GB, used=793.70GB
Data, RAID5: total=579.00GB, used=204.81GB
System, RAID1: total=32.00MB, used=192.00KB
Metadata, RAID1: total=6.00GB, used=3.54GB


Remco

Re: converting to raid5

[David Sterba]

On Mon, Mar 11, 2013 at 09:15:44PM +0100, Remco Hosman wrote:
> first, i did the following: `btrfs val start -dconvert=raid5,usage=1` to convert the mostly empty chunks.
> This resulted in a lot of allocated space (10's of gigs), with only a few 100 meg used.

Matches my expectation, converting to the new profile needs to allocate
full 1G chunks, but the usage=1 filter allows to fill them partially.

After this step, several ~empty raid1 chunks should disappear.

> i did `btrfs val start -dusage=75` to clean things up.
> 
> then i ran `btrfs bal start -dconvert=raid5,soft`.
> I noticed how the difference between total and used for raid5 kept growing. 

Do you remember if this was temporary or if the difference was
unexpectedly big after the whole operation finished?

> My guess is that its taking 1 raid1 chunk (2x1 gig disk space, 1 gig
> data), and moving it to 1 raid5 chunk (4gig disk space, 3gig data),
> leaving all chunks 33% used.

Why 3G of data in raid5 case? I assume you talk about the actually used
data and this should be the same as in raid1 case, but spread over 3x
1GB chunks and leaving them 33% utilized, that makes sense, but is not
clear from your description.

> This is what 3 calls of `btrfs file df /` looks like a few minutes after each other, with the balance still running:

> Data, RAID1: total=807.00GB, used=805.70GB
> Data, RAID5: total=543.00GB, used=192.81GB
> --
> Data, RAID1: total=800.00GB, used=798.70GB
> Data, RAID5: total=564.00GB, used=199.30GB
> --
> Data, RAID1: total=795.00GB, used=793.70GB
> Data, RAID5: total=579.00GB, used=204.81GB

raid1 numbers going down, raid5 going up, all ok.

david

Re: converting to raid5

[Remco Hosman - Yerf-IT]

Op 15-3-2013 13:47, David Sterba schreef:
> On Mon, Mar 11, 2013 at 09:15:44PM +0100, Remco Hosman wrote:
>> first, i did the following: `btrfs val start -dconvert=raid5,usage=1` to convert the mostly empty chunks.
>> This resulted in a lot of allocated space (10's of gigs), with only a few 100 meg used.
> Matches my expectation, converting to the new profile needs to allocate
> full 1G chunks, but the usage=1 filter allows to fill them partially.
>
> After this step, several ~empty raid1 chunks should disappear.
It did not only happen when i added the usage=1, but also without.
>> i did `btrfs val start -dusage=75` to clean things up.
>>
>> then i ran `btrfs bal start -dconvert=raid5,soft`.
>> I noticed how the difference between total and used for raid5 kept growing.
> Do you remember if this was temporary or if the difference was
> unexpectedly big after the whole operation finished?
It did not finish, the filesystem did not have that much space free so i 
canceled it (even before it ran out of space) and ran `btrfs bal start 
-dusage=1` to cleanup the unused space
>> My guess is that its taking 1 raid1 chunk (2x1 gig disk space, 1 gig
>> data), and moving it to 1 raid5 chunk (4gig disk space, 3gig data),
>> leaving all chunks 33% used.
> Why 3G of data in raid5 case? I assume you talk about the actually used
> data and this should be the same as in raid1 case, but spread over 3x
> 1GB chunks and leaving them 33% utilized, that makes sense, but is not
> clear from your description.
I assumed that with raid5, btrfs allocated 1 gig on each disk and uses 1 
for parity, giving 3 gig of data in 4gig diskspace.
>> This is what 3 calls of `btrfs file df /` looks like a few minutes after each other, with the balance still running:
>> Data, RAID1: total=807.00GB, used=805.70GB
>> Data, RAID5: total=543.00GB, used=192.81GB
>> --
>> Data, RAID1: total=800.00GB, used=798.70GB
>> Data, RAID5: total=564.00GB, used=199.30GB
>> --
>> Data, RAID1: total=795.00GB, used=793.70GB
>> Data, RAID5: total=579.00GB, used=204.81GB
> raid1 numbers going down, raid5 going up, all ok.
>
> david