On 15/09/16 11:31, Raghavendra G wrote:
On Thu, Sep 15, 2016 at 12:02 PM, Nithya Balachandran <nbalacha@xxxxxxxxxx <mailto:nbalacha@xxxxxxxxxx>> wrote: On 8 September 2016 at 12:02, Mohit Agrawal <moagrawa@xxxxxxxxxx <mailto:moagrawa@xxxxxxxxxx>> wrote: Hi All, I have one another solution to heal user xattr but before implement it i would like to discuss with you. Can i call function (dht_dir_xattr_heal internally it is calling syncop_setxattr) to heal xattr in dht_getxattr_cbk in last after make sure we have a valid xattr. In function(dht_dir_xattr_heal) it will copy blindly all user xattr on all subvolume or i can compare subvol xattr with valid xattr if there is any mismatch then i will call syncop_setxattr otherwise no need to call. syncop_setxattr. This can be problematic if a particular xattr is being removed - it might still exist on some subvols. IIUC, the heal would go and reset it again? One option is to use the hash subvol for the dir as the source - so perform xattr op on hashed subvol first and on the others only if it succeeds on the hashed. This does have the problem of being unable to set xattrs if the hashed subvol is unavailable. This might not be such a big deal in case of distributed replicate or distribute disperse volumes but will affect pure distribute. However, this way we can at least be reasonably certain of the correctness (leaving rebalance out of the picture). * What is the behavior of getxattr when hashed subvol is down? Should we succeed with values from non-hashed subvols or should we fail getxattr? With hashed-subvol as source of truth, its difficult to determine correctness of xattrs and their values when it is down. * setxattr is an inode operation (as opposed to entry operation). So, we cannot calculate hashed-subvol as in (get)(set)xattr, parent layout and "basename" is not available. This forces us to store hashed subvol in inode-ctx. Now, when the hashed-subvol changes we need to update these inode-ctxs too. What do you think about a Quorum based solution to this problem? 1. setxattr succeeds only if it is successful on at least (n/2 + 1) number of subvols. 2. getxattr succeeds only if it is successful and values match on at least (n/2 + 1) number of subvols. The flip-side of this solution is we are increasing the probability of failure of (get)(set)xattr operations as opposed to the hashed-subvol as source of truth solution. Or are we - how do we compare probability of hashed-subvol going down with probability of (n/2 + 1) nodes going down simultaneously? Is it 1/n vs (1/n*1/n*... (n/2+1 times)?. Is 1/n correct probability for _a specific subvol (hashed-subvol)_ going down (as opposed to _any one subvol_ going down)?
If we suppose p to be the probability of failure of a subvolume in a period of time (a year for example), all subvolumes have the same probability, and we have N subvolumes, then:
Probability of failure of hashed-subvol: p Probability of failure of N/2 + 1 or more subvols: <attached as an image>Note that this probability says how much probable is that N/2 + 1 subvols or more fail in the specified period of time, but not necessarily simultaneously. If we suppose that subvolumes are recovered as fast as possible, the real probability of simultaneous failure will be much smaller.
In worst case (not recovering the failed subvolumes in the given period of time), if p < 0.5 or N = 2 (and p != 1), then it's always better to check N/2 + 1 subvolumes. Otherwise, it's better to check the hashed-subvol.
I think that p should always be much smaller than 0.5 for small periods of time where subvolume recovery could no be completed before other failures, so checking half plus one subvols should always be the best option in terms of probability. Performance can suffer though if some kind of synchronization is needed.
Xavi
Let me know if this approach is suitable.
Regards
Mohit Agrawal
On Wed, Sep 7, 2016 at 10:27 PM, Pranith Kumar Karampuri
<pkarampu@xxxxxxxxxx <mailto:pkarampu@xxxxxxxxxx>> wrote:
On Wed, Sep 7, 2016 at 9:46 PM, Mohit Agrawal
<moagrawa@xxxxxxxxxx <mailto:moagrawa@xxxxxxxxxx>> wrote:
Hi Pranith,
In current approach i am getting list of xattr from
first up volume and update the user attributes from that
xattr to
all other volumes.
I have assumed first up subvol is source and rest of
them are sink as we are doing same in dht_dir_attr_heal.
I think first up subvol is different for different mounts as
per my understanding, I could be wrong.
Regards
Mohit Agrawal
On Wed, Sep 7, 2016 at 9:34 PM, Pranith Kumar Karampuri
<pkarampu@xxxxxxxxxx <mailto:pkarampu@xxxxxxxxxx>> wrote:
hi Mohit,
How does dht find which subvolume has the
correct list of xattrs? i.e. how does it determine
which subvolume is source and which is sink?
On Wed, Sep 7, 2016 at 2:35 PM, Mohit Agrawal
<moagrawa@xxxxxxxxxx <mailto:moagrawa@xxxxxxxxxx>>
wrote:
Hi,
I am trying to find out solution of one
problem in dht specific to user xattr healing.
I tried to correct it in a same way as we are
doing for healing dir attribute but i feel it is
not best solution.
To find a right way to heal xattr i want to
discuss with you if anyone does have better
solution to correct it.
Problem:
In a distributed volume environment custom
extended attribute value for a directory does
not display correct value after stop/start the
brick. If any extended attribute value is set
for a directory after stop the brick the
attribute value is not updated on brick after
start the brick.
Current approach:
1) function set_user_xattr to store user
extended attribute in dictionary
2) function dht_dir_xattr_heal call
syncop_setxattr to update the attribute on all
volume
3) Call the function (dht_dir_xattr_heal)
for every directory lookup in
dht_lookup_revalidate_cbk
Psuedocode for function dht_dir_xatt_heal is
like below
1) First it will fetch atttributes from first
up volume and store into xattr.
2) Run loop on all subvolume and fetch
existing attributes from every volume
3) Replace user attributes from current
attributes with xattr user attributes
4) Set latest extended attributes(current +
old user attributes) inot subvol.
In this current approach problem is
1) it will call heal
function(dht_dir_xattr_heal) for every directory
lookup without comparing xattr.
2) The function internally call syncop xattr
for every subvolume that would be a expensive
operation.
I have one another way like below to correct
it but again in this one it does have dependency
on time (not sure time is synch on all bricks or
not)
1) At the time of set extended
attribute(setxattr) change time in metadata at
server side
2) Compare change time before call healing
function in dht_revalidate_cbk
Please share your input on this.
Appreciate your input.
Regards
Mohit Agrawal
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