[zfs-discuss] Trying to understand zfs RAID-Z

[Steven Sim]

Gurus; I am exceedingly impressed by the ZFS although it is my humble opinion that Sun is not doing enough evangelizing for it. But that's beside the point. I am writing to seek help in understanding the RAID-Z concept. Jeff Bonwick's weblog states the following; " RAID-Z is a data/parity scheme like RAID-5, but it uses dynamic stripe width. Every block is its own RAID-Z stripe, regardless of blocksize. This means that every RAID-Z write is a full-stripe write. This, when combined with the copy-on-write transactional semantics of ZFS, completely eliminates the RAID write hole. RAID-Z is also faster than traditional RAID because it never has to do read-modify-write." I am unable to relate the above statement to the diagram shown in the PDF file 'zfs_last.pdf' entitled "ZFS THE LAST WORD IN FILE SYSTEMS" (also by Jeff Bonwick), on page 11. I was wondering whether Jeff or some one knowledgeable would elaborate further on the above and also answer the following questions; The green and blue "blocks" shown in the diagram on page 11, do the represent actual physical blocks on individual disks or a single RAID-Z stripe write across multiple disks??? The parity for RAID-Z, where is it?? Surely not the checksum stored together in the upper level direct and indirect block pointer? And if not and it is written as a separate block on another disks, then .......I am afraid I do not understand.... Could someone please elaborate more on the statement "Every block is it's own RAID-Z stripe"??? The block being referred to is a single block across multiple disks or a single disk? My sincere apologies if the above questions seem trivial. But I am really struggling to reconcile the statement and the diagram. Warmest Regards Steven Sim Fujitsu Asia Pte. Ltd. _____________________________________________________ This e-mail is confidential and may also be privileged. If you are not the intended recipient, please notify us immediately. You should not copy or use it for any purpose, nor disclose its contents to any other person. Opinions, conclusions and other information in this message that do not relate to the official business of my firm shall be understood as neither given nor endorsed by it.

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