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Why 4k?
- 1. Why 4K? October 2, 2012 George Wilson george.wilson@delphix.com
- 2. Why 4K? (NotY4K) ● This is not the next Millennium bug! Delphix Proprietary and Confidential
- 3. Storage History ● 1998 IBM publishes a paper proposing an increase of disk sector size from 512B to 4K ● 2000 4K IDEMA (International Disk Drive Equipment and Materials Association) committee was formed ● 2005 ZFS released in OpenSolaris with support for block sizes ranging from 512B to 128K ● 2005 512B emulation mode proposed, later known as AF 512e ● 2006 ZFS adds large sector support ● 2009 Advanced Format is approved as naming convention for 4K sectors ● 2011 All hard drive manufactures start to ship AF 512e drives Delphix Proprietary and Confidential
- 4. Advanced Format Drives ● Two flavors of Advanced Format Drives ○ AF 512e - Advanced Format 512B Emulation ○ AF 4Kn - Advanced Format 4K Native Today Future (2012?) Delphix Proprietary and Confidential
- 5. Advanced Format 512e(AF 512e) ● Maps 8 512B logical blocks into 1 physical 4K block ● Provides an emulation layer for compatibility 0 1 2 3 4 5 6 7 512B Logical Blocks 4K Physical Block #0 4K Physical Blocks Delphix Proprietary and Confidential
- 6. Predicting Future Problems "Accesson a 512-byte basis would continue to be supported, but performance would be inferior to that in which access is done on a 4096-byte basis, and might well be inferior to that of previous drives with 512-byte native block size." -- Large Block Size by Paul Hodges and David Cheng, 1998
- 7. Problems in a4K World ● Lies ○ AF 512e Drive lie about their physical block size ○ LUNs from storage vendors lie about their physical block size ● Misaligned I/O ○ Proper partitioning ○ Some AF 512e drives provide an XP jumper (XP partition starts on sector 63, not 4K aligned) ● Read-modify-write Delphix Proprietary and Confidential
- 8. Sub-block Reads Read 512B Block 0 1 2 3 4 5 6 7 512B Logical Blocks 4K Physical Block #1 4K Physical Blocks 0 1 2 3 4 5 6 7 512B Logical Blocks 4K Physical Block #1 4K Physical Blocks Must Read 4K Block Delphix Proprietary and Confidential
- 9. Sub-block Writes (Read-modify-write) LogicalBlock Read 512B 0 1 2 3 4 5 6 7 512B Logical Blocks 4K Physical Block #0 4K Physical Blocks Physical Block Read 4K 0 1 2 3 4 5 6 7 512B Logical Blocks 4K Physical Block #0 4K Physical Blocks Physical Block Write 4K 0 1 2 3 4 5 6 7 512B Logical Blocks 4K Physical Block #0 4K Physical Blocks Delphix Proprietary and Confidential
- 10. Misaligned 4K Writes LogicalBlock Read 4K 0 1 2 3 4 5 6 7 8 9 4K Physical Block #0 4K Physical Block #1 Physical Block Read 2 4K 0 1 2 3 4 5 6 7 8 9 4K Physical Block #0 4K Physical Block #1 Physical Block Write 2 4K 0 1 2 3 4 5 6 7 8 9 4K Physical Block #0 4K Physical Block #1 Delphix Proprietary and Confidential
- 11. Solutions (sort of) ● Override the lies from the device ○ FreeBSD, Illumos, and Linux have all implemented a way to override the discovered sector size ○ FreeBSD ■ using gnop to create 4k device ○ Illumos ■ add an override into sd.conf: sd-config-list = "VENDOR PRODUCT", physical-block-size:4096; ○ Linux ■ zpool create -o ashift=12 tank <device> Delphix Proprietary and Confidential
- 12. Drawbacks of 4Kand ZFS ● Reduced compression ratio ○ Blocks less than 4K mean 0% compression ○ 8K block can only achieve 50% compression ● Migrating drives from 512B to 4K ● Inefficient metadata allocation ○ Some metadata is allocated in 4K chunks and will no longer get compressed ● Improper accounting of compressed sizes in datasets ● RAID-Z and 4k -- not recommended ● Configuring root pools to use 4K ○ Grub support? ● Fewer uberblocks Delphix Proprietary and Confidential
- 13. Q&A / Beer? Delphix Proprietary and Confidential
- 14. ZFS Day October 2, 2012 George Wilson george.wilson@delphix.com