c908dc6f4b
Reviewed by: Christopher Siden <christopher.siden@delphix.com> Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Matthew Ahrens <mahrens@delphix.com> Reviewed by: Don Brady <dev.fs.zfs@gmail.com> Reviewed by: Josef 'Jeff' Sipek <josef.sipek@nexenta.com> Approved by: Garrett D'Amore <garrett@damore.org> Author: Chris Williamson <Chris.Williamson@delphix.com> illumos/illumos-gate@89c86e3229 Currently, every buffer cached in the L2ARC is accompanied by a 240-byte header in memory, leading to very high memory consumption when using very large cache devices. These changes significantly reduce this overhead. Currently: L1-only header = 176 bytes L1 + L2 or L2-only header = 176 bytes + 32 byte checksum + 32 byte l2hdr = 240 bytes Memory-optimized: L1-only header = 176 bytes L1 + L2 header = 176 bytes + 32 byte checksum = 208 bytes L2-only header = 96 bytes + 32 byte checksum = 128 bytes So overall: Trunk Optimized +-----------------+ L1-only | 176 B | 176 B | (same) +-----------------+ L1 & L2 | 240 B | 208 B | (saved 32 bytes) +-----------------+ L2-only | 240 B | 128 B | (saved 116 bytes) +-----------------+ For an average blocksize of 8KB, this means that for the L2ARC, the ratio of metadata to data has gone down from about 2.92% to 1.56%. For a 'storage optimized' EC2 instance with 1600GB of SSD and 60GB of RAM, this means that we expect a completely full L2ARC to use (1600 GB * 0.0156) / 60GB = 41% of the available memory, down from 78%. |
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