Re: ceph-disk improvements

[Loic Dachary <loic@dachary.org>]

Hi Wido,

On 02/04/2016 07:54, Wido den Hollander wrote:
> 
>> Op 1 april 2016 om 17:36 schreef Sage Weil <sweil@redhat.com>:
>>
>>
>> Hi all,
>>
>> There are a couple of looming features for ceph-disk:
>>
>> 1- Support for additional devices when using BlueStore.  There can be up 
>> to three: the main device, a WAL/journal device (small, ~128MB, ideally 
>> NVRAM), and a fast metadata device (as big as you have available; will be 
>> used for internal metadata).
>>
>> 2- Support for setting up dm-cache, bcache, and/or FlashCache underneath 
>> filestore or bluestore.
>>
> 
> Keep in mind that you can't create a partition on a bcache device. So when using
> bcache, the journal has to be filebased and not a partition.

Is this true of all bcache versions ( https://bcache.evilpiepirate.org/ ) ? Or is it a planned feature ? Or is it never going to happen ?

Cheers

> 
> If we add the flag --file-based-journal or --no-partitions we can create OSDs on
> both bcache and dm-cache.
> 
> With BlueStore this becomes a problem since it requires the small (XFS)
> filesystem for it's metadata.
> 
> Wido
> 
>> The current syntax of
>>
>>  ceph-disk prepare [--dmcrypt] [--bluestore] DATADEV [JOURNALDEV]
>>
>> isn't terribly expressive.  For example, the journal device size is set 
>> via a config option, not on the command line.  For bluestore, the metadata 
>> device will probably want/need explicit user input so they can ensure it's 
>> 1/Nth of their SSD (if they have N HDDs to each SSD).
>>
>> And if we put dmcache in there, that partition will need to be sized too.
>>
>> Another consideration is that right now we don't play nice with LVM at 
>> all.  Should we?  dm-cache is usually used in conjunction with LVM 
>> (although it doesn't have to be).  Does LVM provide value?  Like, the 
>> ability for users to add a second SSD to a box and migrate cache, wal, or 
>> journal partitions around?
>>
>> I'm interested in hearing feedback on requirements, approaches, and 
>> interfaces before we go too far down the road...
>>
>> Thanks!
>> sage
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-- 
Loïc Dachary, Artisan Logiciel Libre
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