lkml.org 
[lkml]   [2017]   [Jul]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: bcache with existing ext4 filesystem
On Mon, Jul 24, 2017 at 09:15:48PM +0200, Pavel Machek wrote:
> >
> > Am 24.07.2017 um 20:57 schrieb Pavel Machek:
> > >Would it be feasible to run bcache (write-through) with existing ext4
> > >filesystem?
> > >
> > >I have 400GB of data I'd rather not move, and SSD I could use for
> > >caching. Ok, SSD is connecte over USB2, but I guess it is still way
> > >faster then seeking harddrive on random access
> >
> > i doubt that seriously - USB2 has a terrible latency
>
> Well.. if that's too slow, I can get SSD M.2; plus bcache docs says
> that combination works.
>
> And... if you ever tried to do git diff while git checkout is running
> on spinning rust... spinning rust has awful parameters when idle, and
> it only gets worse when loaded :-(.

So some hard numbers. Max throughput of USB 2.0 is 53 MiB/s[1]. In
actual practice the max throughput you will see out of the USB 2.0
interface is 30-40 MiB/s. In contrast, a HDD doing sequential reads
can easily do much more than that.

[1] https://superuser.com/questions/317217/whats-the-maximum-typical-speed-possible-with-a-usb2-0-drive

So a lot is going to depend on how bcache works. If you can get large
sequential reads and writes to *bypass* the cache device, then I think
there's a good cache that bcache on a USB 2.0 device won't hurt. It
might not help as much as you like, but that's a function of the
overhead of populating the cache and whether the cache can keep the
useful bits in the cache device.

Cheers,

- Ted

\
 
 \ /
  Last update: 2017-07-24 21:28    [W:0.283 / U:0.044 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site