Re: bcache with existing ext4 filesystem

[Pavel Machek <pavel@ucw.cz>]

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Hi!

On Mon 2017-07-24 15:27:18, Theodore Ts'o wrote:
> 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.

Another useful number is that spinning rust does less than 3MB/sec on
common operations done by git. (Yes, probably because a lot of
seeking). So... USB device  should be able to help.

Question for you was... Is the first 1KiB of each ext4 filesystem still
free and "reserved for a bootloader"?

If I needed more for bcache superblock (8KiB, IIRC), would that be
easy to accomplish on existing filesystem?

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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