understanding FC_BY_DIRTY

896445

As per documentation, FC_BY_DIRTY refers to data written only to FlashCache but not synced yet to disk.

While checking in exadata X7 Half Rack noticed FC_BY_DIRTY is having ~10TB per cell server which means ~70TB in FC_BY_DIRTY but I only do have 40TB database hosted. If the data is not synced yet to disk how will it ensure ACID property if database is shutdown.

# dcli -l root -g cell_group cellcli -e "list metriccurrent attributes name,metricvalue where name like \'FC_BY_.*\' "

test07celadm01: FC_BY_ALLOCATED        24,345,586 MB

test07celadm01: FC_BY_ALLOCATED_DIRTY  16,748,864 MB

test07celadm01: FC_BY_ALLOCATED_OLTP   19,140,227 MB

test07celadm01: FC_BY_DIRTY            10,450,509 MB

test07celadm01: FC_BY_STALE_DIRTY      0.000 MB

test07celadm01: FC_BY_USED             21,081,386 MB

test07celadm02: FC_BY_ALLOCATED        24,345,775 MB

test07celadm02: FC_BY_ALLOCATED_DIRTY  16,954,182 MB

test07celadm02: FC_BY_ALLOCATED_OLTP   19,317,459 MB

test07celadm02: FC_BY_DIRTY            10,533,631 MB

test07celadm02: FC_BY_STALE_DIRTY      0.000 MB

test07celadm02: FC_BY_USED             21,008,082 MB

test07celadm03: FC_BY_ALLOCATED        24,345,917 MB

test07celadm03: FC_BY_ALLOCATED_DIRTY  16,909,177 MB

test07celadm03: FC_BY_ALLOCATED_OLTP   19,264,821 MB

test07celadm03: FC_BY_DIRTY            10,515,331 MB

test07celadm03: FC_BY_STALE_DIRTY      0.000 MB

test07celadm03: FC_BY_USED             21,022,858 MB

test07celadm04: FC_BY_ALLOCATED        24,346,150 MB

test07celadm04: FC_BY_ALLOCATED_DIRTY  16,949,356 MB

test07celadm04: FC_BY_ALLOCATED_OLTP   19,316,777 MB

test07celadm04: FC_BY_DIRTY            10,527,517 MB

test07celadm04: FC_BY_STALE_DIRTY      0.000 MB

test07celadm04: FC_BY_USED             21,005,618 MB

test07celadm05: FC_BY_ALLOCATED        24,345,911 MB

test07celadm05: FC_BY_ALLOCATED_DIRTY  16,984,119 MB

test07celadm05: FC_BY_ALLOCATED_OLTP   19,351,014 MB

test07celadm05: FC_BY_DIRTY            10,545,083 MB

test07celadm05: FC_BY_STALE_DIRTY      0.000 MB

test07celadm05: FC_BY_USED             20,995,386 MB

test07celadm06: FC_BY_ALLOCATED        24,346,210 MB

test07celadm06: FC_BY_ALLOCATED_DIRTY  16,979,997 MB

test07celadm06: FC_BY_ALLOCATED_OLTP   19,353,190 MB

test07celadm06: FC_BY_DIRTY            10,542,678 MB

test07celadm06: FC_BY_STALE_DIRTY      0.000 MB

test07celadm06: FC_BY_USED             20,995,334 MB

test07celadm07: FC_BY_ALLOCATED        24,346,126 MB

test07celadm07: FC_BY_ALLOCATED_DIRTY  16,977,600 MB

test07celadm07: FC_BY_ALLOCATED_OLTP   19,364,710 MB

test07celadm07: FC_BY_DIRTY            10,547,420 MB

test07celadm07: FC_BY_STALE_DIRTY      1,860 MB

test07celadm07: FC_BY_USED             20,997,345 MB

Andris Perkons-Oracle

If the data is not synced yet to disk how will it ensure ACID property if database is shutdown.

The DB does not know or care where on the storage server the current data resides. If - from a DB perspective - the data is written, it will either reside on flash or on disk.

The rest is up to the storage server software. It decides whether data has to be written from flash to disk in order to free flash capacity. Flash Cache is persistent across storage server reboots, and ASM redundancy is also maintained regardless of data location.

Andris