Re: [CACHE DEVICE] Space usage

From: gius db <giusdbg@gmail.com>
To: Kai Krakow <kai@kaishome.de>
Cc: linux-bcache@vger.kernel.org
Subject: Re: [CACHE DEVICE] Space usage
Date: Mon, 5 Apr 2021 12:43:33 +0200	[thread overview]
Message-ID: <CAO6aweN7O1uVxauTXzCJQsXoovm3AhiQz1_K0z-b-75iymXZ1g@mail.gmail.com> (raw)
In-Reply-To: <CAC2ZOYsWXUQ+AtQkoXXx50_XSQGMx9xBrgFNP7k0Mt8c79XMcQ@mail.gmail.com>

Hi.

Thanks for the very clear explanation, and the various solutions provided.

My SSD is a 250GB Samsung, I'll do some checking and testing, but I
think I'll leave a 10% free.

Happy holidays.

gdb

Il giorno dom 4 apr 2021 alle ore 21:42 Kai Krakow <kai@kaishome.de> ha scritto:
>
> Hi!
>
> Am So., 4. Apr. 2021 um 18:23 Uhr schrieb Giuseppe Della Bianca
> <giusdbg@gmail.com>:
> > In SSDs, full use of available space causes speed and durability problems.
> >
> > bcahe uses all the available space in the cache device?
> >
> > I could not find information on the maximum space used or how to set it.
>
> There's no option for that in bcache. Instead, create a smaller
> partition for bcache, then create a second partition filling the rest
> of the device. You may want to use a size ratio of 80:20 for these
> partitions tho modern drives usually already have an internal reserve
> area, so 90:10 may be fine, too.
>
> Now, use the blkdiscard command to trim the second partition. That way
> the SSD knows that this is unused space it can use for wear leveling.
> You may remove this second partition if you want to. In either case,
> don't write anything to this space in the future.
>
> Now continue to install bcache to the first partition created.
>
> I've never seen any performance or endurance gains here using modern
> Samsung drives so I've gone with using 100% for bcache. But my older
> smaller drives had seen a benefit (usually better performance than
> better lifetime) from using 80:20 or 90:10. So I'd say the bigger the
> drive, the less likely you need to reserve any trimmed space.
>
> So currently I'm using a hybrid approach and made the second partition
> into a big swap partition: Most of it will stay trimmed but if the
> system has to swap, it will at least find fast swap space here, and it
> can be used for cold hibernation. You should not do that, tho, if your
> system is low on memory: Swap isn't meant as emergency memory, and it
> isn't meant as an extension to installed RAM. It's a space where the
> system can put anonymous memory that's never used to make space for
> disk caching. Only in that case, it's hardly ever written to or read
> from.
>
> Regards,
> Kai