| notes (110592) | notes (110710) |
|---|---|
| 1$FreeBSD: head/sys/geom/notes 110592 2003-02-09 17:04:57Z phk $ | 1$FreeBSD: head/sys/geom/notes 110710 2003-02-11 14:57:34Z phk $ |
| 2 3For the lack of a better place to put them, this file will contain 4notes on some of the more intricate details of geom. 5 6----------------------------------------------------------------------- 7Locking of bio_children and bio_inbed 8 9bio_children is used by g_std_done() and g_clone_bio() to keep track --- 71 unchanged lines hidden (view full) --- 81"wentbusy" timestamp concurrently. 82 83The statistics data is made available to userland through the use 84of a special allocator (in geom_stats.c) which through a device 85allows userland to mmap(2) the pages containing the statistics data. 86In order to indicate to userland when the data in a statstics 87structure might be inconsistent, g_io_deliver() atomically sets a 88flag "updating" and resets it when the structure is again consistent. | 2 3For the lack of a better place to put them, this file will contain 4notes on some of the more intricate details of geom. 5 6----------------------------------------------------------------------- 7Locking of bio_children and bio_inbed 8 9bio_children is used by g_std_done() and g_clone_bio() to keep track --- 71 unchanged lines hidden (view full) --- 81"wentbusy" timestamp concurrently. 82 83The statistics data is made available to userland through the use 84of a special allocator (in geom_stats.c) which through a device 85allows userland to mmap(2) the pages containing the statistics data. 86In order to indicate to userland when the data in a statstics 87structure might be inconsistent, g_io_deliver() atomically sets a 88flag "updating" and resets it when the structure is again consistent. |
| 89----------------------------------------------------------------------- 90maxsize, stripesize and stripeoffset 91 92maxsize is the biggest request we are willing to handle. If not 93set there is no upper bound on the size of a request and the code 94is responsible for chopping it up. Only hardware methods should 95set an upper bound in this field. Geom_disk will inherit the upper 96bound set by the device driver. 97 98stripesize is the width of any natural request boundaries for the 99device. This would be the width of a stripe on a raid-5 unit or 100one zone in GBDE. The idea with this field is to hint to clustering 101type code to not trivially overrun these boundaries. 102 103stripeoffset is the amount of the first stripe which lies before the 104devices beginning. 105 106If we have a device with 64k stripes: 107 [0...64k[ 108 [64k...128k[ 109 [128k..192k[ 110Then it will have stripesize = 64k and stripeoffset = 0. 111 112If we put a MBR on this device, where slice#1 starts on sector#63, 113then this slice will have: stripesize = 64k, stripeoffset = 63 * sectorsize. 114 115If the clustering code wants to widen a request which writes to 116sector#53 of the slice, it can calculate how many bytes till the end of 117the stripe as: 118 stripewith - (53 * sectorsize + stripeoffset) % stripewidth. |
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