From 5212e11fde4d40fa627668b4f2222d20db488f71 Mon Sep 17 00:00:00 2001 From: Vishal Verma Date: Thu, 25 Jun 2015 04:20:32 -0400 Subject: nd_btt: atomic sector updates BTT stands for Block Translation Table, and is a way to provide power fail sector atomicity semantics for block devices that have the ability to perform byte granularity IO. It relies on the capability of libnvdimm namespace devices to do byte aligned IO. The BTT works as a stacked blocked device, and reserves a chunk of space from the backing device for its accounting metadata. It is a bio-based driver because all IO is done synchronously, and there is no queuing or asynchronous completions at either the device or the driver level. The BTT uses 'lanes' to index into various 'on-disk' data structures, and lanes also act as a synchronization mechanism in case there are more CPUs than available lanes. We did a comparison between two lane lock strategies - first where we kept an atomic counter around that tracked which was the last lane that was used, and 'our' lane was determined by atomically incrementing that. That way, for the nr_cpus > nr_lanes case, theoretically, no CPU would be blocked waiting for a lane. The other strategy was to use the cpu number we're scheduled on to and hash it to a lane number. Theoretically, this could block an IO that could've otherwise run using a different, free lane. But some fio workloads showed that the direct cpu -> lane hash performed faster than tracking 'last lane' - my reasoning is the cache thrash caused by moving the atomic variable made that approach slower than simply waiting out the in-progress IO. This supports the conclusion that the driver can be a very simple bio-based one that does synchronous IOs instead of queuing. Cc: Andy Lutomirski Cc: Boaz Harrosh Cc: H. Peter Anvin Cc: Jens Axboe Cc: Ingo Molnar Cc: Christoph Hellwig Cc: Neil Brown Cc: Jeff Moyer Cc: Dave Chinner Cc: Greg KH [jmoyer: fix nmi watchdog timeout in btt_map_init] [jmoyer: move btt initialization to module load path] [jmoyer: fix memory leak in the btt initialization path] [jmoyer: Don't overwrite corrupted arenas] Signed-off-by: Vishal Verma Signed-off-by: Dan Williams --- drivers/nvdimm/btt_devs.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'drivers/nvdimm/btt_devs.c') diff --git a/drivers/nvdimm/btt_devs.c b/drivers/nvdimm/btt_devs.c index effb70a88347..470fbdccd0ac 100644 --- a/drivers/nvdimm/btt_devs.c +++ b/drivers/nvdimm/btt_devs.c @@ -348,7 +348,8 @@ struct device *nd_btt_create(struct nd_region *nd_region) */ u64 nd_btt_sb_checksum(struct btt_sb *btt_sb) { - u64 sum, sum_save; + u64 sum; + __le64 sum_save; sum_save = btt_sb->checksum; btt_sb->checksum = 0; -- cgit