Messages in this thread | | | Date | Wed, 24 Sep 1997 09:35:16 -0400 | From | Chris Wedgwood <> | Subject | Re: safe file systems |
| |
From: Larry McVoy <lm@linux.cobaltmicro.com> To: linux-kernel@vger.rutgers.edu Subject: safe file systems
Do you think it would be possible to build a safe, slow file system? By safe, I mean that I could hit reset in the middle of 50 parallel un-tars and reboot the system and the file system comes up clean (no fsck, but data loss)?
NO fsck - or just a very quick fsck?
And 'hit the reset' is not the only kind of breakage. Do you mean only hit the reset, or do you wish to allow for 'the cat was chewing on the power cable'? (Read below).
Has anyone thought about this very much? If so, is there a mailing list or archive that I can browse?
I thought about it a couple of years back (hmm... actually maybe more) when I was stuffing about with Ultrix. (I was started by writing a defragger, and wanted to change big chunks of the file system around, without sync'ing and still be totally fault tolerant. I never coded a FS, which I'm told for Ultrix is a good thing not to do). It's amazing how easy it is to develop stuff on a modern Linux box :) It would be painful to go back to a machine ten times slower with a fraction on the disk storage and memory....
If you have ordered writes, then surely its possible with no fsck and possibly considerable data-loss... ?
If you wanted a full fsck you could have much lower data-loss. And there are steps in between where a smaller fsck would result in perhaps only moderate data-loss.
I presume the latter is essentially just a journalling file-system.
Also, your writes have to be very strictly ordered. Some intelligent IO subsystems will reorder your write operations on you, which I guess could cause lots of really obscure hard to track down funnies if you were unaware of you IO doing this.
A DPT SCSI card with 64MB of ram (probably with less too) I'm fairly sure does this. (Create a file several GB long, and then update the ends of the file and sync all the time - the HD does not trash). These controllers also delay writes, which for performance is really nice - but in the case of power-loss could be a pain. (Although if writes are ordered it will just mean more data loss that otherwise in this case).
I think even my cheapish SCSI disks (which have a 1 (maybe 2?) MB cache) will do this. I assume here a system reset will not affect them, but power-failure did last time I checked. (But you can twiddle with the tables on them and modify the way it writes data back, etc).
-Chris
| |