[LU-12976] Bigalloc sub cluster allocation for ldiskfs

• Type: Improvement
• Resolution: Unresolved
• Priority: Major
• Labels: ldiskfs
• Assignee: WC Triage
• Reporter: Artem Blagodarenko (Inactive)

Description

For large filesystems (over 256TB) meta_bg is always required, as the 
GDT is larger than a single block group.  However, with bigalloc it 
is possible to avoid meta_bg since the block group size increases by a factor 
of the chunk size as well.  That means a 1PiB filesystem could avoid meta_bg 
if it is using a bigalloc chunk size of 16KB or large. 

Large cluster size is not good for small files, because we need to occupy whole cluster even if we require some blocks only.

There is idea of an interesting project idea that called “sub cluster allocation”. This is “bigalloc” + special bitmap inside bigalloc cluster. This gives less metadata, but small blocks if needed.

Andreas suggesting creating an issue in the jira and start to discuss. Then move discussion to the ext4 email list. I created this issue for initial desussion.

Issue Links

• is related to: LU-12970 improve mballoc for huge filesystems