linux-kernel - Re: Fwd: [PATCH v2] bcache: use bio cloning for detached device requests

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Message-Id: <aXAoi9wTa7VWUiPj@studio.local>
Date: Wed, 21 Jan 2026 09:34:01 +0800
From: "Coly Li" <colyli@...as.com>
To: "Jens Axboe" <axboe@...nel.dk>
Cc: "Stephen Zhang" <starzhangzsd@...il.com>, 
	"Kent Overstreet" <kent.overstreet@...ux.dev>, 
	"Sasha Levin" <sashal@...nel.org>, 
	"Christoph Hellwig" <hch@...radead.org>, <linux-bcache@...r.kernel.org>, 
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>, 
	"zhangshida" <zhangshida@...inos.cn>
Subject: Re: Fwd: [PATCH v2] bcache: use bio cloning for detached device requests

On Tue, Jan 20, 2026 at 08:01:52AM +0800, Jens Axboe wrote:
> On 1/20/26 7:46 AM, Coly Li wrote:
> >> @@ -949,6 +950,11 @@ static int bcache_device_init(struct
> >> bcache_device *d, unsigned int block_size,
> >>                         BIOSET_NEED_BVECS|BIOSET_NEED_RESCUER))
> >>                 goto out_ida_remove;
> >>
> >> +       if (bioset_init(&d->bio_detach, 4,
> > 					^^^^^-> I feel 4 might be a bit small
> > here. bio_detached set is for normal IO when backing device is not
> > attached to a cache device. I would suggest to set the pool size to
> > 128 or 256.
> 
> Absolutely not, 4 is more than plenty. The pool elements are only ever
> used if allocations fail, to guarantee forward progress. Setting aside
> 128 or 256 for that case is utterly wasteful, you only need a couple. 4
> is a good number, if anything it should be smaller (2).

Hi Jens,

Thanks for the information. Please correct me if I am wrong for the following
text,
- If the backing is a normal SSD raid0, the IOPS without attached cache device
might be more than thousands. In this case, I assume 128 or 256 might be more
tolerant.
- I see what ‘4’ means, just not sure/comfortable when memory pressure is high.
And reserving 128/256 will occupy around 0.5~1MB memory, I feel such extra
memory is acceptable in bcache use case.

Don't get me wrong, I totally trust you. If '4' works well enough for high
memory pressure condition for detached bcache device, it is cool.

Thanks in advance.

Coly Li