Am Mon, 15 Jul 2013 19:42:29 +0000 schrieb devel-request@lists.fedoraproject.org:
Message: 5 Date: Mon, 15 Jul 2013 12:25:49 -0700 From: Conrad Meyer cemeyer@uw.edu To: Development discussions related to Fedora devel@lists.fedoraproject.org Subject: Re: F20 System Wide Change: SSD cache Message-ID: 20130715122549.5f14299b@m Content-Type: text/plain; charset=US-ASCII
On Mon, 15 Jul 2013 13:57:39 -0400 DJ Delorie dj@redhat.com wrote:
I think this is a bad idea, at least for my setup. I really don't want my small expensive boot SSD being beaten to death trying to cache a multi-terabyte array, especially since I have plenty of RAM that already serves that purpose (the machine rarely reboots).
Actually, bcache is very good about *not* wearing out SSDs -- it writes in giant erase block-sized portions and likely you can tune how much is written.
And either of these layers must be turned on by an admin -- it's not going to be shoved down your throat.
At the very least, this feature should be disabled if the SSD is the boot/root drive. When SSDs fail, they fail completely, and it's irresponsible to cause early failure on a drive that's critical for booting and OS operation.
By default, bcache runs a write-through cache -- it only caches clean data. If the caching SSD dies, the bcache layer can just forward requests to spinning drive. No data is lost.
(Bcache has a writeback mode where data loss is possible. I do not recommend this mode.)
Also, I think such features should be postponed until/unless there's a clear and obvious way to configure/disable them that doesn't involve installing additional packages or editing obscure text files.
Again -- no one is forcing you to use this. It's opt-in.
Conrad
Hmm, for me me this is more a question how can i configure bcache or dm-cache in a system which use more than one SSD and some conventional hard-disk. Seems like only you or some kernel devs know about how to do this. Can you give a configuration example for f19? For example, i use 4 SSD at a hardware-raid for my several systems (f17/f18/f19/rawhide), i called it A, i need this for testing compiz on baremetal for all current fedora releases. On a second hardware-raid- controller i have 4 conventional disks to store my data, i called it B. So, if i add another SSD for bcache or dm-cache, i called C, how can i configure it? 1. this configuration should not control disk A 2. disk C should use as cache for disk B
Is this possible to configure? Do you have a link with more info's and configuration examples.
best regards, Wolfgang
On Mon, 15 Jul 2013 22:34:15 +0200 Rave it chat-to-me@raveit.de wrote:
Hmm, for me me this is more a question how can i configure bcache or dm-cache in a system which use more than one SSD and some conventional hard-disk. Seems like only you or some kernel devs know about how to do this. Can you give a configuration example for f19? For example, i use 4 SSD at a hardware-raid for my several systems (f17/f18/f19/rawhide), i called it A, i need this for testing compiz on baremetal for all current fedora releases. On a second hardware-raid- controller i have 4 conventional disks to store my data, i called it B. So, if i add another SSD for bcache or dm-cache, i called C, how can i configure it? 1. this configuration should not control disk A 2. disk C should use as cache for disk B
Is this possible to configure? Do you have a link with more info's and configuration examples.
best regards, Wolfgang
First off -- bcache requires an additional superblock on the backing disk / block device -- in this case, B. So you would have to reformat the entire B array if you wanted to use bcache on it.
With that in mind, see: http://evilpiepirate.org/git/linux-bcache.git/tree/Documentation/bcache.txt?...
""" make-bcache -B /dev/path/to/B -C /dev/new/SSD
...
The devices show up as:
/dev/bcache<N>
As well as (with udev):
/dev/bcache/by-uuid/<uuid> /dev/bcache/by-label/<label>
To get started:
mkfs.ext4 /dev/bcache0 mount /dev/bcache0 /mnt """
That document should help answer some of your questions about bcache.
Best regards, Conrad