When we make-bcache on a drive we need to replace the filesytem
superblock with a bcache superblock so the kernel knows to load the
drive through bcache, but this destroys the filesystem. We've talked
about hacky ways to hide the bcache superblock somewhere else, but
it's very dangerous stuff that's likely to fail and we don't want to
support it.
Adam
On Wed, Feb 1, 2012 at 1:44 PM, Piergiorgio Sartor
[off-list ref] wrote:
Hi Adam,
On Wed, Feb 01, 2012 at 01:38:12PM -0800, Adam Berkan wrote:
quoted
Oh, sorry I misunderstood.
You have to run make-bcache once to add a bcache superblock to the drive.
After that the drive contents are destroyed and it needs to be formatted
with a filesystem.
ah! That's not good...
Is there any plan to have the caching device attachable
and detachable from *any* backing device without prior
"formatting" of this second one?
I think bcache is a very interesting and promising
project, but formatting the backing device is
something, I think, that should be avoided.
bye,
pg
quoted
At that point you can attach or detach the drive while it is in use.
Adam
On Wed, Feb 1, 2012 at 12:54 PM, Piergiorgio Sartor <
piergiorgio.sartor-KvP5wT2u2U0@public.gmane.org> wrote:
quoted
Hi Adam,
thanks for the answer, see below.
On Wed, Feb 01, 2012 at 11:04:59AM -0800, Adam Berkan wrote:
quoted
You can attach bcache to a drive with an existing file system, and it
will
quoted
continue as normal. If you connect to a drive without a file system,
then
quoted
it will continue to not have a file system, but you can format it while
attached.
Maybe I misused the term "format".
I did not mean filesystem format, but bcache format.
What I understood, maybe I'm wrong, is that the backing
device, before being used, must be "initialized" with
the bcache tool.
From the docs:
Getting started:
You'll need make-bcache from the bcache-tools repository. Both the cache
device
and backing device must be formatted before use.
make-bcache -B /dev/sdb
make-bcache -C -w2k -b1M -j64 /dev/sdc
I understand this as the backing device gets something
on written on it (note the term "formatted").
Am I wrong? I hope so...
Thanks again,
bye,
pg
quoted
Attach/detach should work while the device is in use. This isn't the
most
quoted
tested code path, especially with writeback on, but it's supposed to
work.
quoted
Detaching while the cache is dirty requires flushing all that data so
performance will be bad until the detach completes.
Let us know if you find any bugs.
Adam
On Wed, Feb 1, 2012 at 2:10 AM, Piergiorgio Sartor <
piergiorgio.sartor-KvP5wT2u2U0@public.gmane.org> wrote:
quoted
Hi all,
first of all I would like to congratulate for this
project, I think it is one of the most promising
feature the Linux kernel can have.
Wrote that, I've a question about the concept of
formatting the backing device.
As far as I understood, the first concept of bcache
was to simply "register" or "attach" a cache to a
backing device, that is, the backing device had not
to be formatted.
Lately, still if I understood it correctly, this
behaviour was changed and, now, the backing device
needs to be formatted.
So, the question is:
How about an already running device? Is it still
possible to attach a cache under such situation?
In general, would it be possible to attach/detach
a cache to any already available device (in the
future)? Or the caching/backing setup must be planned
before the HW is available, so to speak?
It would be useful (and cool too), to have the
possibility to attach/detach the SSD cache, on
the fly (at run-time) to any device it needs it.
I hope the question(s) are clear, if not please
let me know.
Thanks a lot in advance,
bye,
--
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Hi Adam,
On Wed, Feb 01, 2012 at 03:11:54PM -0800, Adam Berkan wrote:
When we make-bcache on a drive we need to replace the filesytem
superblock with a bcache superblock so the kernel knows to load the
drive through bcache, but this destroys the filesystem. We've talked
well, I guess it will destroy the md superblock 1.1 too,
how about LVM metadata?
I think the mismatch is with /dev/bcacheX device.
The first implementation, as far as I remember, was simply
telling the caching device (using UUID) which was the
backing device, i.e. it was registering the backing to
the caching.
Then, still if I got it right, the bcache was caching the
backing device directly, without any need of a third
device (/dev/bcacheX).
I understand that the actual implementation is easier and,
maybe, simpler, since a completely new device is added,
which will have the new caching "features", while the
old one (backing device) is just a further layer.
This is similar to LVM over md over /dev/sdX.
Nevertheless, my opinioni is, while still considering bcache
a great project, that it should work on already existing
devices, without touching them.
Anyway, thanks a lot for the chat,
bye,
pg
about hacky ways to hide the bcache superblock somewhere else, but
it's very dangerous stuff that's likely to fail and we don't want to
support it.
Adam
On Wed, Feb 1, 2012 at 1:44 PM, Piergiorgio Sartor
[off-list ref] wrote:
quoted
Hi Adam,
On Wed, Feb 01, 2012 at 01:38:12PM -0800, Adam Berkan wrote:
quoted
Oh, sorry I misunderstood.
You have to run make-bcache once to add a bcache superblock to the drive.
After that the drive contents are destroyed and it needs to be formatted
with a filesystem.
ah! That's not good...
Is there any plan to have the caching device attachable
and detachable from *any* backing device without prior
"formatting" of this second one?
I think bcache is a very interesting and promising
project, but formatting the backing device is
something, I think, that should be avoided.
bye,
pg
quoted
At that point you can attach or detach the drive while it is in use.
Adam
On Wed, Feb 1, 2012 at 12:54 PM, Piergiorgio Sartor <
piergiorgio.sartor-KvP5wT2u2U0@public.gmane.org> wrote:
quoted
Hi Adam,
thanks for the answer, see below.
On Wed, Feb 01, 2012 at 11:04:59AM -0800, Adam Berkan wrote:
quoted
You can attach bcache to a drive with an existing file system, and it
will
quoted
continue as normal. If you connect to a drive without a file system,
then
quoted
it will continue to not have a file system, but you can format it while
attached.
Maybe I misused the term "format".
I did not mean filesystem format, but bcache format.
What I understood, maybe I'm wrong, is that the backing
device, before being used, must be "initialized" with
the bcache tool.
From the docs:
Getting started:
You'll need make-bcache from the bcache-tools repository. Both the cache
device
and backing device must be formatted before use.
make-bcache -B /dev/sdb
make-bcache -C -w2k -b1M -j64 /dev/sdc
I understand this as the backing device gets something
on written on it (note the term "formatted").
Am I wrong? I hope so...
Thanks again,
bye,
pg
quoted
Attach/detach should work while the device is in use. This isn't the
most
quoted
tested code path, especially with writeback on, but it's supposed to
work.
quoted
Detaching while the cache is dirty requires flushing all that data so
performance will be bad until the detach completes.
Let us know if you find any bugs.
Adam
On Wed, Feb 1, 2012 at 2:10 AM, Piergiorgio Sartor <
piergiorgio.sartor-KvP5wT2u2U0@public.gmane.org> wrote:
quoted
Hi all,
first of all I would like to congratulate for this
project, I think it is one of the most promising
feature the Linux kernel can have.
Wrote that, I've a question about the concept of
formatting the backing device.
As far as I understood, the first concept of bcache
was to simply "register" or "attach" a cache to a
backing device, that is, the backing device had not
to be formatted.
Lately, still if I understood it correctly, this
behaviour was changed and, now, the backing device
needs to be formatted.
So, the question is:
How about an already running device? Is it still
possible to attach a cache under such situation?
In general, would it be possible to attach/detach
a cache to any already available device (in the
future)? Or the caching/backing setup must be planned
before the HW is available, so to speak?
It would be useful (and cool too), to have the
possibility to attach/detach the SSD cache, on
the fly (at run-time) to any device it needs it.
I hope the question(s) are clear, if not please
let me know.
Thanks a lot in advance,
bye,
--
piergiorgio
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quoted
quoted
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
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--
piergiorgio
--
piergiorgio
--
piergiorgio