Re: Add Bcache to an existing Filesystem

[FERNANDO FREDIANI <fernando.frediani@upx.com>]

Thanks for reply Eric.

That sounds quiet dangerous to risk 8TB of production data. I guess the 
only safe way is re-formating the partition and therefore it would mean 
8TB copied-out then copied back in which imposes a longer downtime to 
the server.

Best regards
Fernando


On 26/06/2017 14:58, Eric Wheeler wrote:
> On Mon, 26 Jun 2017, FERNANDO FREDIANI wrote:
>
>> Hello folks.
>>
>> I have been using Bcache in a server in production with pretty good results so
>> far.
>>
>> Now I am looking to add it also to another server that is suffering from disk
>> I/O. However this server in question has a fairly large storage running (circa
>> 8TB of used data) and therefore stopping it for a long period becomes
>> difficult.
>>
>> Question is: Can I add an SSD and Bcache to this server and use it for the
>> existing 8TB filesystem ? If so when I run make-bcache -B /dev/sdx1 to create
>> the backing device will it not overwrite what is currently on /dev/sdx1 ?
>
> Not officially.  The bcache superblock is in the first 8k of disk (by
> default, but can align the payload) so 8TB would need shifted right
> assuming the right-hand-side has room to grow.
>
> Disclaimer: this could be dangerous, so at your own risk:
>
> While I've not tried this, theoretically you could put dm-linear atop of
> the bcache bdev and prepend some metadata area, using the `--data-offset`
> parameter in make-bcache as the start of your data area.  Be sure to test
> on something non-critical if you go this route and make sure the sector
> math is correct or risk damaging the front of your 8TB volume.  If you go
> this route and it works, please report back with what you did!
>
> --
> Eric Wheeler
>
>
>
>> Thanks
>> Fernando
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