HAMMER status WEF March 2009 - outsider viewpoint

Bill Hacker wbh at conducive.org
Thu Mar 5 00:11:12 PST 2009

'Bestoren' again, and cross-posting to the too-seldom-used 
dragonfly.hammer newsgroup set up for porting...

Outsider viewpoint WEF March 5 2009, hopefully renderd 'stale' soonest:

- Desired:

-- ability to roll-out hammerfs in pilot, if not full-production use for 
its excellent and inherent fast snapshot/rollback features.

- Current Barriers:

-- limited/no ability to enforce quotas / prevent overflow damage.

-- little/no ability to export sub-dirs as coherent 'chunks', at least 
with hammerfs-specific tools.  'rsync' and cpdup' still work as always.
or nearly so, as ...

-- above complicated by limited 'awareness' of hammerfs specifics from 
the viewpoint of some of the legacy tools:


- 'cpdup' can manage softlinks 'as directed', but scp -r cannot, and may 
expand several snapshots onto a target.

- 'ls' does not always act as expected, nor return useful info, nor the 
*same* info if PFS mounts are / are not involved. Likewsie, to a lesser 
extent 'du'.

Other 'traditional' tools are similarly challenged, may benefit from, OR 
be even more confused by use of PFS mounts.

Needed: more hammerfs-specific alternatives and/or hammerfs awareness 
integration. Neither expected overnight.

Workarounds planned for the moment..

- maintain the 'system' on traditional UFS where traditional tools act 
as they always have. Backing that up is a road well-traveled, versioning 
can live elsewhere.

- Slice separately-mounted large media for client storage use into 
smaller-than-optimal sizes for hammerfs. This to reduce the risk of one 
client overflowing and damaging the storage area of another. Sub-optimal 
sizing ain't the same as 'useless', but an overfilled disk IS, and has 
been known to be able panic the system undr at least a few edge-case 

How so: A 1TB or 1.5TB drive sliced into 50 to 500 GB portions, 
typically 100-250GB, for the working storage.

- Use of a more optimal entire-device hammerfs for the target of hammer 
mirror-copy/hammer mirror-stream backup.

Premise is that the limits on the source will insure that the target has 
no overflow issues. Or 'fewer' anyway.

Seen to be needed 'Real Soon Now':

- 'pluggable' back-end transport choices to hammer mirror-copy / hammer 
mirror-stream for clustering/ mirroring, to wit:

-- 'raw' Ethernet GigE and 10GigE for local - even roll-cable - use 
within a rack. The local SAN isolation/trust model, no need or TCP/IP or 
ssh overheads. More akin to the old dual-controller SCSI chains. 
Significant throughput improvement should be possible w/o erroring 
becoming a problem.

-- Infiniband 'verb' drivers (See Glusterfs)

-- iSCSI, eSATA over Ethernet, fibre channel - whatever else can be 
adapted or implemented 'soonest' and most easily. SSA and FCAL can 
probably be let for dead...

Side Note:

The 'fuse' approach to (involved fs of choice) doesn't look to ever be 
much more than a parlour-trick handy for maintenance. Bonnie++ or 
blogbench too easily drop it in its tracks vs even a basic legacy NFS 
mount. All the more so if either/both source and target happen to be 
running anything even the least bit hungry in userspace (Xorg and 
friends, even if idle).

No need to take that detour.

Hope and trust some food for thought comes out of this.

First we walk. THEN we run....


Bill Hacker

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