Re: [Gluster-devel] EAGAIN/EBUSY handling in glusterfs

[Shishir Gowda]

The lvm snap scenario was taken as an example.

The question I raised was whether we need to handle these errors?

With regards,
Shishir

----- Original Message -----
From: "Anand Avati" <address@hidden>
To: "Shishir Gowda" <address@hidden>
Cc: address@hidden
Sent: Thursday, January 24, 2013 12:18:42 AM
Subject: Re: [Gluster-devel] EAGAIN/EBUSY handling in glusterfs




On Wed, Jan 23, 2013 at 1:34 AM, Shishir Gowda < address@hidden > wrote: 


Hi Avati, 

One of the possible scenarios is someone taking a lvm snap of the backend. 




Can you describe in more detail exact operations for which LVM snap returns 
EAGAIN or EINTR? EINTR in posix is best retried in posix level. However I'm not 
sure if LVM snapshote actually makes the disk filesystem return these non 
standard errors for any reason. Can you give an example strace of this 
happening? 


Avati 


few eg: 
DHT's rebalance: we would not retry a migration if case we got an error EAGAIN 
or even EINTR. 
Does self-heal retry healing if the error was EAGAIN or EINTR? 

These are just few I can think about. 

When snap feature becomes supported (refer to wiki link in previous page), few 
ops' would be blocked while snap is in progress. 

If we decide to provide complete snap in the future (not just 
crash-consistent), then in all probability all fops will be blocked. 

Do we guarantee all op's(triggered internally) that fail will be re-triggered? 
Or are we guaranteeing a state from which we can recover completely? 

With regards, 
Shishir 


----- Original Message ----- 
From: "Anand Avati" < address@hidden > 
To: "Shishir Gowda" < address@hidden > 
Cc: address@hidden 
Sent: Wednesday, January 23, 2013 1:23:09 PM 
Subject: Re: [Gluster-devel] EAGAIN/EBUSY handling in glusterfs 




On Tue, Jan 22, 2013 at 10:39 PM, Shishir Gowda < address@hidden > wrote: 


Hi All, 

Currently I see that almost all the xlators in glusterfs do not handle 
EAGAIN/EBUSY errors. 

Though this should be handled by the applications, 


If by "handle by application" you meant "handled by retrying syscall by 
application", that is not completely true. More generally it is true for EINTR, 
and some places for EAGAIN (i.e when used on non-blocking pollable file 
descriptors like sockets - which specifically does NOT include filesystem for 
regular read/write). EBUSY almost always does not suggest a poll/retry to the 
application. 


there are multiple paths where the op's are not performed by the applications 
(but are internal to glusterfs). 

Few of these are 
a. Rebalance 
b. Replace brick 
c. Self-heal 
d. lk's 
etc... 

With the proposed snap feature ( 
http://www.gluster.org/community/documentation/index.php/Features/snapshot ), 
would it not be better to identify such op's inside glusterfs? 






Can you explain more on that? Why is that necessary? 


Thanks, 
Avati 


Irrespective of the snap feature, I think it is about correctness to handle 
EAGAIN/EBUSY in these code paths. 

Please comment. 

With regards, 
Shishir 


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