[lustre-discuss] Interpretation of tot_granted, tot_pending, tot_dirty stats

[Andreas Dilger]

On Apr 6, 2025, at 05:18, Rasool Almasikoupaei via lustre-discuss <lustre-discuss at lists.lustre.org> wrote:

Hi everyone,


Just wondering if anyone knows what these metric values actually mean and how I can make sense of them:

obdfilter.testfs-OST0000.tot_granted
obdfilter.testfs-OST0000.tot_pending
obdfilter.testfs-OST0000.tot_dirty

Appreciate any help!

The "space grant" mechanism controls how much dirty data can be cached on the clients (per OST) before the writes are throttled.  This avoids clients dirtying GB (or TB these days) of data in RAM but the OST is actually out of space and cannot write the data to persistent storage.

tot_granted = total amount of space granted to all clients for this OST
tot_pending = total amount of dirty data in process of being written (i.e. in RPCs)
tot_dirty = total amount of dirty data reported by clients

The clients send these values to the server with every write RPC, so the server knows the client totals even before the write RPCs are sent.  This allows the server to pre-emptively send more write grants to a client so that it doesn't need to throttle writes, if there is still lots of free space on the server.

Cheers, Andreas
—
Andreas Dilger
Lustre Principal Architect
Whamcloud/DDN




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