I've set up server with big amount of disk space available (10x10Tb HDDs). This server should deliver (over HTTP) files to many clients, usual file size is several Mb. I use RAID 6 and XFS. I make a decision to make chunk size as large as possible. My reasoning is: HDD performance is mostly limited by seeks. With default chunk size (512Kb) reading of 4Mb file touches 8 HDDs (8 seeks) With large chunk size only one HDD is touched (1 seek). So I created array with maximal possible chunk size (16Mb). And I have an issues with this array. https://bugzilla.kernel.org/show_bug.cgi?id=201331 I have another server with similar setup. I did some tests on it. As expected large chunk size provides significantly better multithreaded large block read performance. But write performance drops with chunk size over 4Mb. So I set up second server with chunk size 4Mb. And I have no such deadlocks with this server. Now I tried do change chunks size on the first server, but no success: # mdadm --grow /dev/md3 --chunk=4096 --backup-file=/home/md3-backup chunk size for /dev/md3 set to 16777216 (and no changes in /proc/mdstat) I have some questions: 1. Is deadlock under load an expected behavior with 16Mb chunk size? Or it is a bug and should be fixed? 2. Is it possible to reshape existing RAID with smaller chunk size? (without data loss) 3. Why chunk size over 4Mb causes bad write performance?
