Re: [PATCH 00/14 RFC] Btrfs: Add journal for raid5/6 writes

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-08-23 11:28, Chris Murphy wrote:
> On Wed, Aug 2, 2017 at 2:27 PM, Liu Bo <bo.li.liu@oracle.com> wrote:
>> On Wed, Aug 02, 2017 at 10:41:30PM +0200, Goffredo Baroncelli wrote:
> 
>>> What I want to understand, is if it is possible to log only the "partial stripe"  RMW cycle.
>>>
>>
>> I think your point is valid if all data is written with datacow.  In
>> case of nodatacow, btrfs does overwrite in place, so a full stripe
>> write may pollute on-disk data after unclean shutdown.  Checksum can
>> detect errors but repair thru raid5 may not recover the correct data.
> 
> What's simpler? raid56 journal for everything (cow, nocow, data,
> metadata), or to apply some limitations to available layouts?
> 
> -  if raid56, then cow only (no such thing as nodatacow)
This should obviously be something that will be contentious to certain 
individuals.
> -  permit raid56 for data bg only, system and metadata can be raid1, or raid10
> 
> I'm hard pressed thinking of a use case where metadata raid56 is
> beneficial over raid10; a metadata heavy workload is not well suited
> for any parity raid. And if it isn't metadata heavy, then chances are
> you don't even need raid10 but raid1 is sufficient.
Until BTRFS gets n-way replication, raid6 remains the only way to 
configure a BTRFS volume to survive more than one device failure.
> 
> Of the more complicated ways to solve it:
> 
> - journal
> - dynamically sized stripes, so that writes can always be full stripe
> writes, no overwrites, and atomic
> - mixed block groups where only sequential full stripe writes use
> raid56 block group; random and smaller writes go in a raid 1 or 10
> block group.
>