> So, filestore:
> - write data to the journal
> - write metadata
> - move data from journal to definitive storage
>
> Bluestore:
> - write data to definitive storage (free space, not overwriting anything)
> - write metadata
> - write data to the journal
> - write metadata
> - move data from journal to definitive storage
>
> Bluestore:
> - write data to definitive storage (free space, not overwriting anything)
> - write metadata
Wait, doesn't that mean that when I move to Bluestore from Filestore I'll have increased latency for writes?
Currently I'm using filestore with SSD journals, and my write latency is that of SSD. By that logic in Bluestore my writes will come straight to HDD, and I'll have HDD write latency?
2017-09-20 12:37 GMT+03:00 Burkhard Linke <Burkhard.Linke@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>:
Hi,
On 09/20/2017 11:10 AM, Sam Huracan wrote:
So why do not journal write only metadata?
As I've read, it is for ensure consistency of data, but I do not know how to do that in detail? And why BlueStore still ensure consistency without journal?
The main reason for having a journal with filestore is having a block device that supports synchronous writes. Writing to a filesystem in a synchronous way (e.g. including all metadata writes) results in a huge performance penalty.
With bluestore the data is also stored on a block devices, and thus also allows to perform synchronous writes directly (given the backing storage is handling sync writes correctly and in a consistent way, e.g. no drive caches, bbu for raid controllers/hbas). And similar to the filestore journal the bluestore wal/rocksdb partitions can be used to allow both faster devices (ssd/nvme) and faster sync writes (compared to spinners).
Regards,
Burkhard
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