[LU-9290] max_pages_per_rpc can't be smaller than ZFS recordsize

• Type: Bug
• Resolution: Unresolved
• Priority: Major
• Affects Version/s: Lustre 2.9.0
• Environment: Lustre 2.9.0; ZFS based OSTs
• Severity: 3
• Assignee: Nathaniel Clark
• Reporter: Erich Focht

Description

At a customer we've hit LU-5718 and were trying to set max_pages_per_rpc to a value lower than 256. This didn't work. Reading the source showed:

 

lustre/obdclass/lprocfs_status.c: osc_obd_max_pages_per_rpc_seq_write()

 

chunk_mask = ~((1 << (cli->cl_chunkbits - PAGE_CACHE_SHIFT)) - 1);
/* max_pages_per_rpc must be chunk aligned */
val = (val + ~chunk_mask) & chunk_mask;
if (val == 0 || (ocd->ocd_brw_size != 0 &&
                 val > ocd->ocd_brw_size >> PAGE_CACHE_SHIFT)) {
        LPROCFS_CLIMP_EXIT(dev);
        return -ERANGE;
}

chunkbits is 20. It is set in lustre/osc/osc_request.c:osc_init_grant() to

cli->cl_chunkbits = max_t(int, PAGE_SHIFT, ocd->ocd_grant_blkbits);

and ocd_grant_blkbits is set to 20. It's comment line says: /* log2 of the backend filesystem blocksize */

Once I've reduced the ZFS recordsize from 1MB to 512kB and remounted the OST, I
was able to reduce the max_pages_per_rpc.

I believe that the value set in ocd_grant_blkbits is wrong, and actually should be the ZFS ashif value (i.e. the block size) and not the recordsize.