[Xen-devel] OCaml xenstore: max value size is configure to be 2048 bytes - consistent with C xenstore?

[Christian Lindig <christian.lindig@xxxxxxxxxx>]

We recently had a case where XenServer installation showed errors because the 
data stored in OCaml xenstore exceeded 2048 bytes:

[root@dt87 tmp]# xenstore-write /local/domain/1/foo $(cat /dev/zero | tr '\0' X 
| dd bs=1 count=2048)
2048+0 records in
2048+0 records out
2048 bytes (2.0 kB) copied, 0.00743674 s, 275 kB/s
[root@dt87 tmp]# xenstore-write /local/domain/1/foo $(cat /dev/zero | tr '\0' X 
| dd bs=1 count=2049)
2049+0 records in
2049+0 records out
2049 bytes (2.0 kB) copied, 0.00714459 s, 287 kB/s
xenstore-write: could not write path /local/domain/1/foo


This limit is configured in quota-maxsize:

https://github.com/mirage/xen/blob/master/tools/ocaml/xenstored/oxenstored.conf.in#L50

This could be a surprise because in quota.ml it is initialised to 4096 and 
later reset when the config file is read.

https://github.com/mirage/xen/blob/master/tools/ocaml/xenstored/quota.ml#L24

My questions: is this behaviour consistent with the C xenstore implementation 
and if not, should this be documented or changed? Should the OCaml 
implementation made more consistent by using the 2048 for the initial value?

— Christian


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