journal: set max journal write to 10MB

If we take too big a bite of data to write in a single writev(2), we can
end up making performance worse, because everyone waits for the full write
to complete.  Bigger writes mean better throughput but higher latency.
So, balance the two by placing some upper limit.

diff --git a/src/config.cc b/src/config.cc
index 4215f5ceb1557d75fbb4dc6051e6026532de7b6c..fea938a123b79ffa002c4ecaff1721c468bf409b 100644 (file)
--- a/src/config.cc
+++ b/src/config.cc
@@ -500,7 +500,7 @@ static struct config_option config_optionsp[] = {
        OPTION(ebofs_verify_csum_on_read, 0, OPT_BOOL, true),
        OPTION(journal_dio, 0, OPT_BOOL, true),
        OPTION(journal_block_align, 0, OPT_BOOL, true),
-       OPTION(journal_max_write_bytes, 0, OPT_INT, 0),
+       OPTION(journal_max_write_bytes, 0, OPT_INT, 10 << 20),
        OPTION(journal_max_write_entries, 0, OPT_INT, 100),
        OPTION(journal_queue_max_ops, 0, OPT_INT, 500),
        OPTION(journal_queue_max_bytes, 0, OPT_INT, 100 << 20),