On 12/16/2014 03:21 PM, J. Bruce Fields wrote:
On Tue, Dec 16, 2014 at 11:46:46AM -0500, Shyam wrote:
On 12/15/2014 09:06 PM, Anand Avati wrote:
Replies inline
On Mon Dec 15 2014 at 12:46:41 PM Shyam <srang...@redhat.com
<mailto:srang...@redhat.com>> wrote:
With the changes present in [1] and [2],
A short explanation of the change would be, we encode the subvol ID in
the d_off, losing 'n + 1' bits in case the high order n+1 bits of the
underlying xlator returned d_off is not free. (Best to read the commit
message for [1] :) )
Although not related to the latest patch, here is something to consider
for the future:
We now have DHT, AFR, EC(?), DHT over DHT (Tier) which need subvol
encoding in the returned readdir offset. Due to this, the loss in bits
_may_ cause unwanted offset behavior, when used in the current scheme.
As we would end up eating more bits than what we do at present.
Or IOW, we could be invalidating the assumption "both EXT4/XFS are
tolerant in terms of the accuracy of the value presented
back in seekdir().
XFS has not been a problem, since it always returns 32bit d_off. With
Ext4, it has been noted that it is tolerant to sacrificing the lower
bits in accuracy.
i.e, a seekdir(val) actually seeks to the entry which
has the "closest" true offset."
Should we reconsider an in memory _cookie_ like approach that can help
in this case?
It would invalidate (some or all based on the implementation) the
following constraints that the current design resolves, (from, [1])
- Nothing to "remember in memory" or evict "old entries".
- Works fine across NFS server reboots and also NFS head failover.
- Tolerant to seekdir() to arbitrary locations.
But, would provide a more reliable readdir offset for use (when valid
and not evicted, say).
How would NFS adapt to this? Does Ganesha need a better scheme when
doing multi-head NFS fail over?
Ganesha just offloads the responsibility to the FSAL layer to give
stable dir cookies (as it rightly should)
Thoughts?
I think we need to analyze the actual assumption/problem here.
Remembering things in memory comes with the limitations you note above,
and may after all, still not be necessary. Let's look at the two
approaches taken:
- Small backend offsets: like XFS, the offsets fit in 32bits, and we are
left with another 32bits of freedom to encode what we want. There is no
problem here until our nested encoding requirements cross 32bits of
space. So let's ignore this for now.
- Large backend offsets: Ext4 being the primary target. Here we observe
that the backend filesystem is tolerant to sacrificing the accuracy of
lower bits. So we overwrite the lower bits with our subvolume encoding
information, and the number of bits used to encode is implicit in the
subvolume cardinality of that translator. While this works fine with a
single transformation, it is clearly a problem when the transformation
is nested with the same algorithm. The reason is quite simple: while the
lower bits were disposable when the cookie was taken fresh from Ext4,
once transformed the same lower bits are now "holy" and cannot be
overwritten carelessly, at least without dire consequences. The higher
level xlators need to take up the "next higher bits", past the previous
transformation boundary, to encode the next subvolume information. Once
the d_off transformation algorithms are fixed to give such due "respect"
to the lower layer's transformation and use a different real estate, we
might actually notice that the problem may not need such a deep redesign
after all.
Agreed, my lack of understanding though is how may bits can be
sacrificed for ext4? I do not have that data, any pointers there
would help. (did go through https://lwn.net/Articles/544520/ but
that does not have the tolerance information in it)
Here is what I have as the current bits lost based on the following
volume configuration,
- 2 Tiers (DHT over DHT)
- 128 subvols per DHT
- Each DHT instance is either AFR or EC subvolumes, with 2 replicas
and say 6 bricks per EC instance
So EC side of the subvol needs log(2)6 (EC) + log(2)128 (DHT) +
log(2)2 (Tier) = 3 + 7 + 1, or 11 bits of the actual d_off used to
encode the volume, +1 for the high order bit to denote the encoding.
(AFR would have 1 bit less, so we can consider just the EC side of
things for the maximum loss computation at present)
Is 12 bits still a tolerable loss for ext4? Or, till how many bits
can we still use the current scheme?
To a first approximation, assume ext4's offsets are random numbers (it's
actually some kind of weak cryptographic hash). So in a directory with
n entries, you're basically picking n random numbers out of a hat and
hoping you never pick the same number twice.
So following preshing.com/20110504/hash-collision-probabilities, the
chance that two entries in an n-entry directory will have the same b-bit
cookie are (for small n/2^b), roughly
n^2 / 2^(b+1)
I think 52 bits is still a lot. Even on million-entry directories I
only get a one-in-ten-thousand chance. But play with the numbers and
make sure I haven't messed up somewhere.
Well I did the (different) math on the following premise, and here is
what I have,
For even distribution of hashes in a given range
n hash values generated (directory entries)
N maximum value in the hash range
s Separation distance of 2 values, (N/n)
b potential bits lost, but separation still preserved, floor(log(s)(base
2))
Basically as we are losing some bits to our encoding (lower bits of the
d_off), given a 'k' (as the d_off) finding the nearest hash that it
could represent, would mean losing less than 1/2 the bits of the
distance separating 2 hash values (assuming even distribution of hash
values). Which is what 'b' is above.
With this and N set to represent a 64 bit range, we would need > ~16
million entries, before losing 20 bits of information starts mattering.
Considering this, it seems safe enough against ext4 (assuming the hash
function that is used has the property of value distribution evenly
across the range (at least to a point)) and assuming we even grow to 1/2
a million bricks in a gluster volume (~2^19).
Alert: Above math needs checking and verification
Anyway, a longer-term approach might be to fix the NFS protocol's
reliance on stable directory cookies, but of course then you have to
wait till the NFS clients get upgraded.
Also, other file systems may choose to use the d_off cookie differently,
but I assume that is handled when it appears on the horizon.
--b.
Thanks,
Shyam
If we move to 1000/10000 node gluster in 4.0, assuming everything
remains the same except DHT, we need an additional 3-5 bits for the
DHT subvol encoding. Would this still survive the ext4 encoding
scheme for d_off?
Hope that helps
Thanks
Shyam
[1] http://review.gluster.org/#/c/__4711/
<http://review.gluster.org/#/c/4711/>
[2] http://review.gluster.org/#/c/__8201/
<http://review.gluster.org/#/c/8201/>
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