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Re: [RFC] directory quota survey on xfs

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC] directory quota survey on xfs
From: Zheng Liu <gnehzuil.liu@xxxxxxxxx>
Date: Tue, 21 Jan 2014 15:07:06 +0800
Cc: Andreas Dilger <adilger@xxxxxxxxx>, Ext4 Developers List <linux-ext4@xxxxxxxxxxxxxxx>, Theodore Ts'o <tytso@xxxxxxx>, Dmitry Monakhov <dmonakhov@xxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, xfs@xxxxxxxxxxx, Li Xi <pkuelelixi@xxxxxxxxx>
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Mail-followup-to: Dave Chinner <david@xxxxxxxxxxxxx>, Andreas Dilger <adilger@xxxxxxxxx>, Ext4 Developers List <linux-ext4@xxxxxxxxxxxxxxx>, Theodore Ts'o <tytso@xxxxxxx>, Dmitry Monakhov <dmonakhov@xxxxxxxxxx>, Ben Myers <bpm@xxxxxxx>, xfs@xxxxxxxxxxx, Li Xi <pkuelelixi@xxxxxxxxx>
References: <20131222095929.GA11444@xxxxxxxxx> <20131223014222.GC3220@dastard> <20140115081201.GA3820@xxxxxxxxx> <5966C533-EF18-4804-8C83-DFBD607D9BE6@xxxxxxxxx> <20140115213207.GJ3431@dastard>
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Hi Dave,

On Thu, Jan 16, 2014 at 08:32:07AM +1100, Dave Chinner wrote:
> On Wed, Jan 15, 2014 at 11:03:22AM -0700, Andreas Dilger wrote:
> > On Jan 15, 2014, at 1:12 AM, Zheng Liu <gnehzuil.liu@xxxxxxxxx> wrote:
> > > On Mon, Dec 23, 2013 at 12:42:22PM +1100, Dave Chinner wrote:
> > >> I'd also strongly suggest that you use the XFS userspace quota API
> > >> for managing project quotas, so that we can use the same management
> > >> tools and tests to verify that they behave the same. Please don't
> > >> invent a new version of the quota API to implement this - everything
> > >> you need ifor managing project/directory quotas is already there in
> > >> xfs_quota.....
> > > 
> > > Frankly, I don't like this, really.  Now we have quota-tool to manage
> > > the quota in ext4.  So IMHO we'd better go on using this tool because it
> > > is natural for ext4 users.
> 
> Zheng - you're confusing the userspace tool that users run with
> the quotactl API the tool uses to communicate with the kernel.

Thanks for pointing it out.

> 
> > > I still couldn't accept this fact that I
> > > need to install xfsprogs for using a feature of ext4.  Further, it could
> > > make users puzzled because they use quota to control user/group quota in
> > > ext4, but it uses xfs_quota to control project quota.  It could bring
> > > some troubles for the ext4 users who have written some scripts to manage
> > > their machines.
> > 
> > Please see Li Xi's recent email "Directory/Project quota supports" on
> > the linux-ext4 list.  He has already added some prototype support for
> > project quotas to quota-tools.
> 
> So, while it is a prototype, lets do it the right way. i.e. let's
> not reinvent the wheel.

Yes, I agree with you and Andreas that we shouldn't reinvent the wheel.

> 
> > I think it might make sense to keep the same API as XFS for the ext4
> > quotas (to keep compatibility for existing XFS deployments), but add
> > support into quota-tools so that it is usable by all filesystems.
> 
> Well, yes. If you are writing a generic quota tool, then it needs to
> support all filesystems. We already have a fully featured quota API
> that can provide this support - it's the API that XFS has been using
> since it was ported to Linux.  We have the opportunity to unify the
> quota APIs that ext4 and XFS, so we should take the opportunity
> while it is here.  Don't create a new API for ext4 simply because of
> NIH syndrome.

These days I was thinking about your comment that uses quotactl API to
communicate the userspace tool with the kernel.  But I am still
confusing about your comment that unifies the quota API between ext4 and
XFS.

Now we have two flag sets in quotactl(2).  One (Q_QUOTAON, Q_GETQUOTA,
etc...) is used by extN file system (I am not sure whether other file
systems use these flags or not), and another (Q_XQUOTAON, Q_XGETQSTAT,
etc...) is used by XFS.

In xfs_quota it uses an ioctl(2) to get/set/check project id, and calls
quotactl(2) with Q_XSETQLIM/Q_XGETQUOTA to set/get project quota.  On
kernel side, ->set_dqblk()/get_dqblk() is called when we try to set/get
project quota in XFS.  In ext4 the same callback functions are used to
set/get user/group quota, although on userspace we use Q_SETQUOTA/
Q_GETQUOTA to set/get quota.  I am not sure I fully understand your
meaning that unifies the quota API between ext4 and XFS.  Do you mean
that we should use Q_XSETQLIM/Q_XGETQUOTA flags to set/get quota on ext4?
Or using quotactl(2) is fine for you.

Please correct me if I miss something.

Thanks,
                                                - Zheng

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