Solaris
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The quota or rq options in /etc/vfstab are only used by quotaon -a at boot
time, to determine whether to turn on quotas for the filesystem.
When quotas are activated, the quota option is added to /etc/mnttab. Actually
mounting with this option is pointless - only the quotaon command can really
turn on quotas.

When quotas are newly enabled for a filesystem and a user assigned block
limits (but not file), these limits seem to take effect immediately. 
When the file limits are turned on, they also take effect immediately
(but the number of existing files does not seem to be computed).

On the other hand, blocks used are always recorded even if the quotas
are set to 'unlimited'.

Linux
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The usrquota and grpquota options in /etc/fstab are used by quotaon
whenever it is run. Unlike solaris, the filesystem must exist in the fstab
file (and have quota options) for quotaon to do anything. Also, quotaon does
not update the mtab file when quotas are turned on (but does complain if 
quotas are activated twice). Similarly, quotaoff does nothing unless the
quota options are given in /etc/fstab
The quota option documented in the man pages seems to be totally ignored - 
only usrquota and grpquota are used by quotaon.

Quotas seem to have some problems under redhat linux using kernel 2.0.21
(redhat 5.1). quotacheck and repquota will often hang when both user and
group quotas are active at the same time. Each can be used separately
with no problems though (as long as a quotacheck is done after creating
the quotas file!)

When quotas are enable for some filesystem, they take effect immediately for
both files and blocks. Even if a user creates files and then has quotas
set, the counts will be up to date.