Message-ID: <20100427204254.GC5103@quack.suse.cz>
Date:	Tue, 27 Apr 2010 22:42:55 +0200
From:	Jan Kara <jack@...e.cz>
To:	tytso@....edu
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: DRAFT Design Spec for 1st Class Quota Support in Ext4

On Mon 26-04-10 18:57:58, tytso@....edu wrote:
> On Mon, Apr 26, 2010 at 08:47:47PM +0200, Jan Kara wrote:
> >   Hi Ted,
> > 
> >   The proposal looks fine. A couple of comments / questions:
> > 
> > > 6) Tune2fs will have a facility for adding and removing user and group
> > > quotas inodes while the file system is mounted.  The quota usage will
> > > not be correct after the quota inodes are newly added, however, so quota
> > > will not be enabled by default,  If the quota inodes are removed, quota
> > > will be disabled first.
> >   I suppose you will make quota file mtime not match last mounted time so
> > that e2fsck updating the usage will be triggered, right?
> 
> Yes, that was my original plan (when adding quotas to a mounted
> filesystem).  The one downside to this is that it requires rebooting
> and running e2fsck in order enable quotas, which is kind of unfortunate.
> 
> One thing which I was thinking about doing was running quotacheck and
> having it create aquota.user,group files the old-fashioned (admittedly
> racy) way, and then having tune2fs zap those inodes into the
> superblock and then enabling quotas.  The usage counters won't be
> exactly write if the filesystem has any activity happening while
> quotacheck si running, but that's a problem system administrators seem
> to be willing to live with today.
  When all the needed functionality for gathering quota information will
be in e2fsck already, wouldn't it be more natural to use e2fsck for
gathering quota information on mounted filesystem as well? Or what would
be benefit of using quotacheck for this? It uses libext2fs to do inode
scan anyways...

> We can then have e2fsck convert the aquota.{user,group} files to
> hidden files by simply copying the inode to inodes 3 and 4, and then
> removing the link in the root directory (without requiring a full fsck
> run) on the next reboot.  We use a similar trick today to migrate from
> a /.journal file to a hidden journal inode in e2fsck already, so most
> of that code is in e2fsck already.
> 
> > > 7) There will be a new interface so that bulk quota information can be
> > > fetched from the file system.  This needs to be negotiated with Jan
> > > Kara.  It can either be a new system call, or a magic file in /proc that
> > > can be opened and the repquota data extracted.
> >   Traditionally, quota interaction happens via quotactl() syscall. I'm not
> > against adding a syscall for this but then it would be a logical thing to
> > transition all the quotactl interface to standard syscalls... I'll try to
> > come up with some proposal during this week.
> 
> I'm OK with whatever you can get past the LKML bike shed painting
> crew.  :-)
> 
> There's a set of folks who tend to whinge about how multiplexed system
> calls are evil, and I'm not sure whether they will complain about
> adding more multiplexing to the quotactl system call, with the
> attendent need for 64->32 compat complexity.
> 
> >   Using /proc/ file looks like an ugly hack to me.
> 
> Well, unlike the other the other quotactl subcommands, this one is
> going to require some kind of first/next iterator, since the number of
> user/groups that needs to be returned is highly variable.  And in
> fact, if we need a system call per user/group data that we need to
> extract, it's going to be somewhat painful from a syscall overhead
> point of view, isn't it?
> 
> Using a single /proc file means many fewer syscalls to read out the
> repquota data, since userspace can use a nice big 4k buffer.  (I
> suppose the repquota quotactl subcommand could fill in a big passed-in
> array, but that's more complexity.)  And with a /proc file, userspace
> would be able to read out the repquota information directly using a
> shell script.
> 
> But I don't have strong preference if you really want to use a binary
> format instead of a /proc file.
  So what I have in mind is that we would have QUOTASCAN_OPEN quotactl
which would create a fd for the caller (essentially a read end of the pipe)
and return it. Reading from the fd would give quota information as if_dqblk
structures. The read would return only complete if_dqblk structures.
Internally I'd use f_pos to store the id (uid / gid) of the next structure
to return to maintain scan state. This should be reasonably flexible,
efficient, and clean interface. What do you think?

> >   And one more question: You don't speak in your document, when quotas
> > will be enabled. I expect accounting will be enabled during mount time, how
> > about quota enforcement? I think it might be useful if admin can disable
> > the quota limit enforcement and QUOTAON / QUOTAOFF sysctl is a natural
> > way to do this. Generic quota code supports this so it comes basically
> > for free.
> 
> I was assuming that it was better to simply enable quota enforcement
> by default; how often do system administrators want to selecting
> enable and disable quota?  It seemed to me that a huge amount of the
> hair and complexity and nastiness of the /etc/init.d/quota scripts was
> because quota checking had to be enabled from userspace, and there was
> a need to grovel through /etc/mtab (or /proc/mounts) to figure out
> which file systems needed to have quota enabled, not just for
> quotacheck.
> 
> I consider the amount of mount options that we have to pass through to
> /proc/mounts to be horrible, and one of my primary design goals was to
> eliminate all of that.  So my plan was to enable quotas enforcement
> and tracking when the file system is mounted.
  Well, mount options are a separate issue. As soon as filesystem is
aware of potential quotas since mount, we know that Q_GETFMT quotactl
will return quota format ID (instead of -ESRCH) if quota accounting is
started. So quota-tools can use this and there's no need for special
mount options.

> Is there a usage scenario where a system administrator would want to
> mount a file system and _not_ enforce quota?
  Hmm, no, I'm not aware of any reasonable scenario. So feel free to
enable enforcement on mount.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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